Analytical laser ablation of solid samples for ICP, ICP-MS, and FAG-MS analysis

ABSTRACT

This invention is improved laser ablation of solid samples analyzed by inductively coupled plasma (ICP), ICP mass spectrometry, or flowing afterglow mass spectrometry. A mirror-with-hole eliminates chromatic aberration in sample viewing and allows rad-hardening for radiation hot cell analysis of nuclear waste. Other attributes facilitate comprehensively rad-hardened laser ablation. Additional improvements include large, homogeneous laser spots, long-focus objective lenses, variable laser path length with built-in re-alignment, variable demagnification ratio, higher powered SMR lasers with larger spots enabling sensitive bulk solids analysis, demountable gravitationally self sealing stack assembly sample ablation cells, and laser ablation sample changers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a C.I.P. of U.S. application Ser. No. 12/283,698,filed Sep. 14, 2008 (scheduled to issue Sep. 25, 2012 as U.S. Pat. No.8,274,735) and therevia claims benefit of U.S. Provisional ApplicationSer. Nos. 60/993,795 and 61/134,136 filed on Sep. 14, 2007 and Jul. 7,2008, respectively. Said application Ser. No. 12/283,698 and said U.S.Provisional Applications No. 60/993,795 and 61/134,136 are herebyincorporated by reference.

FIELD OF THE INVENTION

This invention relates to 1.) improved optical viewing of ultraviolet(hereafter “UV”) laser ablation processes involving solid materials, andto 2.) high sensitivity analysis of solid materials by analytical UVlaser ablation, and to 3.) large spot bulk analysis of solid materialsby analytical UV laser ablation, and to 4.) automated (mechanized)sample changing for analytical laser ablation, and to 5.) lasermicro-machining, and to 6.) large depth-of-focus laser ablation forablating rough, uneven or non-level surfaces, and to 7.) wide-range,variable demagnification ratio laser ablation, and to 8.) rad-hardenedanalytical UV laser ablation for analysis of high activity solid nuclearwaste (e.g. vitrified as radioactive glass) in a radiation “hot cell”,

in which a focused UV laser beam removes (by optical ablation at focusedlaser energy densities (more precisely, irradiance) exceeding thesurface damage threshold of said solid material) a portion from thesurface of said solid materials, for purposes of altering the shape ortopography of said solid materials, or for purposes of obtaining vapors,or smoke, or a particulate aerosol from a laser ablation event occurringon the surface of said solid material, or obtaining a mixture of vapors,smoke, and/or particulate aerosol from a laser ablation event occurringon the surface of said solid material, which vapors, smoke, orparticulate aerosol, or which mixture of vapors, smoke, and/orparticulateaerosol may then be directed to an external analytical instrument (e.g.inductively coupled plasma (ICP) emission, ICP mass spectrometer(ICP-MS), or flowing after-glow (FAG) mass spectrometer (FAG-MS), saidexternal analytical instrument being capable of providing chemicaland/or elemental analysis of said vapors, or smoke, or particulateaerosol, or said mixture of vapors, smoke, and/or particulate aerosol,said chemical and/or elemental analysis being indicative andrepresentative of the chemical or elemental content and/or compositionof the original said solid sample materials.

The invention therefore relates to UV laser ablation micro-machining(e.g. in an industrial setting), and/or to analytical UV laser ablationin a general solid sample analysis laboratory or a radiation “hot cell”environment,

and more specifically, the invention relates to 1.) improved opticalviewing (by an observer or camera) of the solid material surface to beablated, and/or during ablation, and/or post-ablation in UV laserablation, via a reduction of chromatic aberration to allow an observer(or camera) to view (more clearly) the area to be ablated, to view (moreclearly) the process of ablation, and/or view (more clearly) the resultof ablation, in such a way as to allow an observer to better identifyand/or pre-select the area to be ablated, to more clearly view andvideo-record the ablation event in order to assess ablation processcharacteristics, and/or to more clearly view the result of ablation onthe solid material surface to assess ablation effectiveness and ablationcrater or trench morphology and compare that with previous or futureablation experiments in same or other materials under same or variedablation conditions, said observation being typically opticallymagnified and then viewed by direct human visual inspection through amagnified optical eyepiece, or a video-camera, or CCD array camera whichmay be used to capture and/or record images for immediate (real-time)display on a monitor screen, and/or for storage in a computer file,and further, the invention specifically relates to 2.) high sensitivityanalytical UV laser ablation, 3.) large spot bulk analysis in analyticalUV laser ablation, and 4.) laser micro-machining, respectively in whichsubstantially larger (than normal) invention UV lasers may be employedin an invention stable multimode resonator (SMR) lasing conditioncoupled with an invention external optical configuration for larger SMRlasers which results in an inherently homogenized near field inventionlaser beam profile being efficiently transferred without homogeneityloss to a far field sample surface, which results in externalsubstantially larger invention material ablation rates in these threeareas (enumerated above) without increasing the average particle size ofsmoke or aerosol resulting from the ablation, and creating a morehomogeneous focused high powered laser spot capable of ablating moreflat bottomed craters and trenches, without deteriorating the “quality”of ablation craters and/or trenches produced (in terms of crater (and/ortrench) edge cut (sharp, clean edges without cracking, chipping, orshattering) and/or bottom shape (e.g. bottom flatness)),and further, the invention specifically relates to 5.) exceptionallylarge depth-of-focus laser ablation in focusing a laser-light image of alaser illuminated aperture (hereafter laser spot) onto the surface of atarget solid sample or target material for ablating rough, non-flat,non-parallel, non-level, or otherwise uneven target surfaces tofacilitate tolerance of surface variations (from flat and level) aslarge as 1-2 mm for either stationary laser spot ablation or line scanablation and/or raster pattern ablation without refocusing, re-leveling,or resurfacing (e.g. grinding flat) the target sample or material.and further, the invention specifically relates to 6.) wide-rangevariable demagnification invention laser ablation, in which largechanges in invention optical demagnification of the laser spot may bemade on an operational basis in a single invention laser ablation systemto optimize ablation rates, aerosol quality, and or crater and/or trenchsize and quality within the ideal irradiance range (IIR) of eachmaterial and for a wide variety of different solid materials,and further, the invention specifically relates to 7.) automatic(mechanized) solid sample changing for analytical laser ablation,and finally, the invention specifically relates to 8.) rad-hardenedinvention analytical laser ablation of solid sample materials in aradiation “hot cell” containing radioactive sample material such asnuclear waste, in which optical components (of the invention laserablation system) that may otherwise be prone to radiation damage areeliminated from the invention design, or are located outside the hotcell (but in adjacent proximity to the hot cell), or are shielded withinthe hot cell, or are located within the hot cell at greater distancefrom radioactive material (and hence receive reduced radiation levels bythe “inverse square” law).

BACKGROUND OF THE INVENTION

A description of laser ablation in the publication of Arrowsmith andHughes, APPLIED SPECTROSCOPY, 42, 7, 1988 (1231-1239) is commonly citedas the beginning of modern analytical laser ablation for inductivelycoupled plasma (ICP) emission and inductively coupled plasma massspectrometry (ICP-MS) analysis of solid samples. More recently, the July2008 issue of Gases & Instruments features an article by Hughes, Brady,and Fry which reviews the use of UV lasers and analytical laser ablationin general, with illustration of how white light illumination andviewing is normally done for UV laser ablation and discussion ofparameters affecting ablation quality, ablation morphology, ablationrate, and related aerosol particle size from an ablation event. From theG&I article it should be noted that an opto-mechanical (OM) ablation isdesired for analytical laser ablation, rather than a thermal process. Itshould also be noted from the G&I article that a small particle size isdesired in the aerosol resulting from an ablation event, to ensureefficient aerosol mass transport (to the external analytical instrument)and to minimize overall system calibration difficulty and variability.

FIGS. 1A, 1B illustrate that a dichroic mirror (6) is normally used inprior art analytical UV laser ablation, to allow a view camera (22) toview a solid material (11, 24) coaxially (18, 19, 20) with a finalsegment of the UV laser beam (7, 10, 24) which is also focused to ablatethe surface of the solid material (11, 24). The prior art dichroicmirror (6) has a very thin film mirror coating which is highlyreflective only to light of a specific UV laser wavelength, which is thespecific “design” wavelength of the particular (dichroic) laser mirrorin question, and which is based on selective constructive interference(in the reflection mode) of light at that design wavelength,exclusively. All other wavelengths (shorter and longer than the specificUV design wavelength) are not reflected. Instead, the thin mirrorcoating is transparent to the other wavelengths (e.g. visible light) andpasses them like a window (even if angled). FIGS. 1A, 1B therebyillustrate that the prior art UV laser beam (5, 7) may be efficientlyreflected from the angled side of an appropriately designed UV dichroicmirror (6) towards a solid sample surface (11, 24), while an overheadvisible “white light” camera (20, 22) view may be taken through the sameprior art angled UV dichroic laser mirror (6) from the top, since the UVdichroic laser mirror (6) is transparent to visible (e.g. “white”)light.

The prior art objective lens (8) performs two functions. First itfocuses the UV laser beam (7, 10) downward onto the solid samplematerial surface (24); second, it simultaneously operates (in reverse)to coaxially focus a visible, white light image of the solid samplesurface (24) upward (18, 19, 20) to the camera (22) focal plane. (Itshould be noted that an auxiliary visible, white light source, e.g. ringilluminator (16) is typically also provided to coaxially illuminate (17)a relatively wide area (e.g. 1-10 mm) of the solid sample surface (11,24) continuously (to light the “subject” for the camera view), while thepulsed, Q-switched UV laser fires (flashes) intermittently (10), butrepetitively to ablate a smaller spot (24, e.g. 0.02-0.2 mm) on thesolid sample surface.)

The disadvantage of the prior art coaxial camera view of laser ablationin FIGS. 1A, 1B is that both the laser beam (7, 10) and the camera view(18, 19, 20, 22) must pass through the same short focal length prior artobjective lens (8), albeit in opposite directions, so the two prior artoptical paths are (undesirably) coupled. The prior art objective lens(8) must be designed to efficiently pass (7, 10) UV laser radiation(e.g. 193 nm, 213 nm, and 266 nm) with high transparency at UVwavelengths. The available optical materials (for doing that) do notsimultaneously allow an ideally achromatic focused visible (e.g. 400nm-700 nm) “white light” view for the prior art camera path (18, 19, 20,22). The prior art UV laser objective lens (8) can produce a goodquality monochromatic UV laser image (24), but then it is not achromaticfor longer wavelength visible light and therefore cannot focus both endsof the white light spectrum (red and blue) simultaneously in the sameprior art camera plane (22). Undesirable chromatic aberration thusarises for the prior art “white light” on-axis view, in order to ensurea good UV laser ablation experiment. A poorly focused prior art “whitelight” (camera) image (22) therefore adversely affects many analyticalUV laser ablation systems today, and there remains a need to decouplethe visible white light camera view (22) from the chromatic aberrationof a UV laser objective lens (8).

A second disadvantage of prior art UV analytical laser ablation is thatlow ablation rates and poor sensitivity for bulk solid analysistypically result under conditions where a high quality opto-mechanical(hereafter OM) ablation occurs with prior art UV analytical laserablation systems. The basic problem arises from a situation where UVanalytical laser ablation (for ICP and ICP-MS) is a relatively newfield, with complete (integrated) prior art analytical systems becomingcommercially available for the first time in 1995. With UV analyticallaser ablation still in its “infancy” (a relatively small number ofinstallations as of this writing), prior art commercial analytical laserablation manufacturers are both small in size and few in number. Thusfar (1995-present), the small group of prior art analytical laserablation manufacturers have primarily been designing prior art productstailored to the needs of a narrowly focused group of customers workingin Geology.

Geologists have certainly done the infant analytical laser ablationfield a significant service by purchasing prior art commercial unitsearly in its manufacturing development cycle, thereby making the infantfield of analytical laser ablation commercially viable (albeit on arelatively small commercial scale). Through effective lobbying, they(geologists) have influenced the small group of prior art analyticallaser ablation manufacturers and successfully imposed their ownparticular (geologic) biases onto the features and characteristics ofcommercially available prior art system configurations. The few existingprior art analytical laser ablation manufacturers have therefore cateredprimarily to the (prior art) geologic “configuration bias” (hereafter,“geo-bias”), rather than designing flexible, general purpose analyticallaser ablation systems of the type that would be needed for widespreadusage for bulk analysis of solid materials in general, for a widervariety of laboratories.

The prior art geo-bias typically dictates a small, homogeneous focusedspot diameter, since geologists are typically interested in elementalanalysis of small inclusions and other small heterogeneities in rocksand minerals. Consequently, focused laser spot diameters as small as 2micron are desired in the geo-bias, and prior art excimer and SMRanalytical UV laser ablation systems are not sold with a homogeneousfocused spot diameter larger than 200 microns. Prior art SMR analyticalUV laser ablation objective lenses (8, FIG. 1A-1B) to produce such smallhomogeneous spot diameters typically exhibit short focal length (e.g.F=18-38 mm) and their working distance (to the sample surface) is onlyslightly more than that. This prior art geo-bias for short focal lengthobjective lenses and small spot diameters is ideal for geologistsinterested in analyzing small isolated features in heterogeneous rocksand minerals, but it is not ideal for high sensitivity bulk solidsanalysis or more homogeneous sample materials in other fields.

The short focal length objective lens and small spot diameterscharacteristic of the geo-bias in prior art SMR analytical laserablation, actually preclude using high laser power to enhancesensitivity. In fact, there is a certain maximum laser power that can beoptimally employed for prior art focused laser spots of 200 micronsdiameter and less, which is the largest homogeneous focused spotavailable in commercial prior art excimer and SMR analytical laserablation systems. In prior art analytical laser ablation manufacturing,the geo-bias therefore leads to use of relatively small excimer and SMRUV lasers (less than 12 mJ at 266 nm) and short laser path lengths. Thiskeeps the system size and price down, but it also limits the sensitivitywhich can be obtained in bulk solids analysis with a prior art system.

At ETH-Zurich, Guenther, Horn, and Guillong employed larger prior artGaussian beam (TEM 00) lasers with external prior art beam homogenizingoptics, and a large excimer laser was substituted in a commercial priorart system (Geo-Lase by Coherent, distributed for several years byCETAC), but in both cases, the external beam homogenizers wereinefficient (subject to significant light transmission loss), the firingfrequency reduced to 10 Hz maximum for the TEM 00 Nd-YAG laser, and theobjective lenses were characterized by the short geo-bias focal length(F<40 mm) and relatively small maximum focused spot diameter in bothcases, so the actual final output (relating to ablation rate) of theseprior art systems was only slightly more than the smaller, moreefficient, prior art frequency-multiplied SMR Nd-YAG analytical UV laserablation systems, and sensitivity for bulk analysis wasn't appreciablyenhanced with either of these two larger prior art lasers and theirassociated ablation systems.

One larger (40 mJ) prior art commercial Nd-YAG laser ablation systemoperating at 266 nm, 10 Hz was coupled to a maximum focused spot size of780 μm (0.78 mm), but this prior art laser ablation system (MACRO by NewWave, Inc.) wasn't designed for operation in the SMR mode to produce ahomogenized beam profile. It was instead an unstable multimode resonator(UMR) with a gradient reflectance mirror (GRM), by design. The prior artGRM unstable resonator is actually designed for small spot focusing (lowdivergence rate, compared to SMR) and it is well known that the GRMunstable multimode resonator (UMR) does not produce the desirablehomogenized beam profile for large spots, and is instead characterizedby a “donut with hole” or “scooped” beam profile. Initial laser ablationtesting with this prior art GRM unstable resonator (UMR) analyticallaser ablation system determined that it was not a reliableconfiguration at high power (40 mJ, 266 nm). This prior art unstableresonator (UMR) deteriorated rapidly in terms of power output andablation crater quality. In summary, the GRM unstable multimoderesonator (UMR) beam profile is not homogeneous like an SMR, and thelimited prior art firing frequency of 10 Hz further reduces thesensitivity of a MACRO system relative to 20 Hz SMR system. Finally, theenergy output of this unstable resonator has been reported to be erraticand frequently dropping to 20 mJ instead of the 40 mJ UMR rating.

There remains a need for high sensitivity analytical UV laser ablationbased on a stable, reliable, high powered (e.g >12 mJ @ 266 nm, withsimilar higher powered 213 nm and 193 nm systems) homogenous beam SMR(stable multimode resonator) laser with a firing frequency higher than10 Hz and a laser objective lens with focal length greater than F=40 mmwith reduced demagnification to produce larger homogeneous focused spotdiameters (>200 um) commensurate with higher laser power to achieve highanalytical sensitivity within the ideal irradiance range (IIR) of solidsample materials.

The referenced G&I article indicated that each different solid samplematerial has a relatively narrow range of focused laser irradiance(joules/cm²/ns) which is ideal for producing the best OM ablationcharacteristics. Operating within the ideal irradiance range (IIR) for agiven material minimizes thermal ablation effects (which otherwise makecalibration more difficult and unreliable) and yields the smallestaerosol particle size. If the focused laser ablation irradiance is lowerthan the IIR for a given material, then thermal ablation predominates,ablation rates are low, calibration is difficult and unreliable, andanalytical sensitivity is poor. If the focused laser irradiance ishigher that the IIR of a material, then that sample is “over powered”and undesirable sample shattering and cracking occurs, destabilizing theanalytical instrument response without significantly improving thesensitivity. In this case, ablation is too violent (rough) and too manylarge particles are blown out of the ablation crater, the largeparticles being too large for efficient transport to the externalinstrument. They wind up splattered throughout the ablation cell,settling out on various cell and tubing wall surfaces withouttransporting to the plasma or contributing appreciably to the analysis.In such an overpowered situation, the signal in the external instrumentbecomes temporally unstable. The ideal irradiance range (IRR) shouldtherefore be maintained for each material and should not be exceeded.

For small focused spot diameters (<200 urn) characteristic of thegeo-bias, the IIR is matched with relatively small, low powered SMR UVlasers and relatively short prior art laser paths and short focal lengthprior art objective lenses. For example, for a 266 nm (4^(th) harmonic)pulsed, Q-switched, Nd-YAG laser, SMR systems in the range of 9-12 mJare about the limit of useful laser size, in prior art commercialsystems where the geo-bias prevails to limit the maximum focused priorart spot diameter to 200 microns or less. Larger lasers of 30 mJ, 40 mJ,50 mJ, 60 mJ, 90 mJ, and 230 mJ are available at 266 nm and the desiredSMR mode, but these have typically not been used for prior artanalytical laser ablation, simply because the geo-bias prevailing inthat industry precludes their usage in prior art short path applicationswith a focused spot size range 2-200 microns, where they would simplyover-power the ideal irradiance range (IIR) of virtually all solidsamples.

The overall result of favoring smaller lasers, shorter path lengths, andlimited spot diameter (geo-bias in the prior art analytical laserablation industry) is that prior art system sizes and prices are“contained”, but analytical sensitivities in this prior artconfiguration are limited to the part-per-million (ppm) range for ICPand ICP-MS analysis of solids. There is no reported prior art highsensitivity (part-per-billion, ppb) analytical UV laser ablation systembased on a stable multi-mode resonator (SRM) and which produceshomogeneous focused spot diameters up to 1.5 mm (in a preferredembodiment) and allows use of pulsed SMR 266 nm lasers as large as 50mJ-230 mJ in a long path length configuration, or other equivalentlyoversized UV lasers at even shorter wavelength, while still operatingwithin the optimized IIR of solid materials.

An invention is therefore needed for analytical UV laser ablation inwhich the ppm (part-per-million) sensitivity limitations of the priorart short laser path, short objective focal length, high demagnificationratio and small spot geo-bias would be removed via replacement with amore sensitive analytical laser ablation invention employing longerlaser path lengths and longer focal length objective lenses in a ratiofavoring lower demagnification ratios and larger spot diameters fromlarger SMR lasers operating at full power, coupling most of their energyinto the sample without exceeding IIR values of solid materials to beanalyzed. This would enhance the sensitivity of bulk analysis byanalytical laser ablation and lead to a new era of high sensitivity (ppb(part-per-billion)) bulk analysis in the solid phase. It would befurther desirable if this were achieved simultaneously with theaforementioned invention decoupling of laser focusing from white lightfocusing.

Since the ideal irradiance range (IIR) varies widely in solid materials,but is a relatively narrow range for each material, it is apparent thatconventional systems with fixed demagnification ratio have a limitedability to maintain the IIR of each material in a wide range of solidmaterials, while simultaneously running the system at 100% laser powerand using the full laser beam to maximize ablation rates. If a largerlaser were selected, optical attenuation or power attenuation could beemployed to “throttle it back” and keep all samples within theirrespective IIR's, but if the demagnification ratio is fixed as withprior art systems, ablation rates will not be kept at the maximumpossible ablation rate for that laser over a wide range of solid samplematerials.

There remains a need for an invention which would allow wide range,operationally variable demagnification ratio (operationally variablemaximum spot diameter), so that the laser may be operated at 100% poweroutput while the ablation proceeds within the IIR of each solid materialby simply having the spot diameter adjusted so that 100% of the laserpower is delivered within the IIR of that material. This couldtheoretically be done to a limited extent with a turret holding 2 to 4different interchangeable objective lenses to yield several differentdemagnification ratios, but the number and range of focal lengths whichmay be accommodated in a single turret (for a fixed turret-to-laser head“object” distance and a limited range of turret-to-sample image distancevariation) is limited to about 3 or 4 lenses whose focal lengths are notwidely varying (one from the other). The IIR of solid materials variesmore widely than an objective lens turret could cover by itself. Inorder to accommodate a wider range of IIR, an invention with anoperationally variable laser “object” distance (over wide range) and anoperationally variable laser “image” distance (over wide range) is alsoneeded (or needed instead). Essentially, there remains a need for alaser ablation system with operationally variable path (over a largerange of path length) to create a larger range of demagnification ratiosfor each objective lens. Such an invention would benefit both analyticallaser ablation and laser micro-machining applications.

A third disadvantage of prior art analytical laser ablation is that thecurrently prevailing geo-bias involving relatively short prior art laserpath lengths and short prior art laser objective focal lengths yields ashallow depth-of-focus (in the focused prior art laser spot) of onlyabout 0.25 mm or less. If the sample surface roughness, topography,flatness, or deviation from parallel (to ablation cell horizontaltranslation axis when mounted in cell) varies by more than this,different locations on the sample surface must be refocused uponchanging location in a prior art system. For a laser ablation line scanor raster pattern involving controlled (motorized, FIG. 1B, items 47-52)horizontal sample motion (during ablation), it is obvious that thesample flatness (and degree to which the sample surface is held parallelto the axis of motion) must be less than the depth-of-focus of the laserspot doing the ablation, otherwise the spot will lose focus and theablation rate will change during the horizontal motion scan or raster onthe sample surface. This typically means that the sample must be flatand parallel to the motion axis, within 0.25 mm (250 μm) or less in aprior art system, and it often requires that solid samples with surfaceroughness or uneven topography (greater than this) must be cut or groundflat, prior to ablation.

As one of the principal advantages of laser ablation (compared to aciddissolution of solid samples prior to ICP or ICP-MS analysis withnebulizer introduction of the resulting liquid) is supposed to be“elimination of sample preparation”, the oft-required cutting, grinding,or pelletizing of irregular surfaced solid samples for conventionalprior art laser ablation is clearly counter-productive. There remains aneed for an analytical laser ablation invention with increaseddepth-of-focus (in the focused laser spot) from the current prior artrange of 0.25 mm (or less) to a much larger invention depth-of-focussuch as 1 mm or even 2 mm, to accommodate greater surface roughness andlarger variation in surface topography for laser ablation analysiswithout prior sample preparation or resurfacing by cutting, grinding, orpelletizing.

A fourth disadvantage to prior art laser ablation is the lack of anautomated sample changer, which (lack) prevents automated sequentialanalysis of a large group of samples, or even a small group of samplesif they are too large for more than one of them to fit into the ablationcell at any one time. Many reasons preclude the use of an auto-samplerin prior art analytical laser ablation. For one example, the short focallength prior art objective lenses (geo-bias) typically do not allow roomfor the sample cell to be automatically opened while positioned underthe objective lens. There remains a need for development of an automatic(mechanized) sample changer for analytical laser ablation.

A fifth disadvantage of prior art analytical laser ablation is that, inthe field of high activity nuclear waste analysis, prior art analyticalUV laser ablation has heretofore not been well suited to a radiation“hot cell” environment, due to rapid prior art laser ablation componentfailure upon exposure to high level radioactivity. Typical keyconventional prior art laser ablation component failures occur within500-1,000 rads total accumulated exposure. With high activity nuclearwaste samples in a hot cell, exposure rates of 1,000-2,000 rads/hour areto be expected. This means key conventional prior art laser ablationcomponents would fail within 1 hour or less, and sometimes within 15minutes. This is true of prior art small motors, optical coatings,electronic circuits—especially integrated circuits, laser heads, powersupplies, sensors, and video cameras. Additional prior art componentssubject to failure on a somewhat longer time scale (still problematic)include cables, connectors, insulation on wires, o-rings, lubricants,adhesives, and a variety of plastic or polymer parts, as well asconventional optics. Laser mirrors (thin film dichroic) are particularlysusceptible to radiation damage. Conventional prior art analytical UVlaser ablation systems can't even withstand 1 day in the hot cell withhigh activity nuclear waste samples which nevertheless require analysis.

In a March 2007 government report (07-DESIGN-042, U.S. DOE Office ofRiver Protection, contract DE-ACO5-76RL01830), the US DOE has designatedlaser ablation as a critical technology element (CTE) necessary for the$12.3B nuclear waste processing (vitrification) plant now underconstruction at the DOE Hanford, WA site. It would therefore bedesirable if an invention comprehensively rad-hardened laser ablationsystem could be developed to withstand 1,000-2,000 rads per hour for anexpected useful life of 7-12 years in that environment, instead offailing within less than a day, or less than 1 hour. A total radiationtolerance of 100 million rads total accumulated exposure is thereforedesired for an invention comprehensively rad-hardened laser ablationsystem for nuclear waste analysis. With prior art UV laser ablationsystems failing within 500-1,000 rads total accumulated exposure, it isclear that there remains a need for a new invention to meet DOEradiation hot cell needs.

SUMMARY OF THE INVENTION

The invention laser ablation system of FIGS. 2A, 2B replaces aconventional prior art dichroic mirror beam combiner (((6)—FIG. 1)coaxially combining the final segment of the laser optical path with theinitial segment of a white light viewing system) with an inventionangled mirror-with-hole (25, 26)—FIGS. 2A, 2B, 9A, 9B. This allows aninvention focused UV laser beam (7, 10) to pass (unaltered) through thehole (26) forming a focused spot on the solid material surface (11, 24)below, while the invention observer “white light view” (18-20, 22) ofsaid solid material surface is obtained with the invention angled mirrorperimeter (33) concentrically surrounding the hole (26) and said UVlaser beam (7, 10) passing through said hole (26), and said white lightview obtained in an area encompassing and concentrically surroundingsaid focused laser spot. The advantage of this invention is optical“decoupling” of the UV laser beam (7, 10) from the “white light”observer view (18, 20, 22, 28), even though the two invention lightpaths coaxially share a superimposed path segment (10, 18).

Invention optical decoupling of the two paths is desirable for UV laserablation to allow separate optical optimization of the invention laserpath and the invention white light camera view. (A prior art “coupledpath” does not permit this). The invention UV laser objective lens (8)focuses only the UV laser light, and does not affect the invention whitelight camera view which may then be separately focused with an inventionhigh quality achromatic visible lens (21), optimized separately for theinvention camera (22), and thereby eliminating chromatic aberration fromthe camera view (22, 28). This invention allows the best laser ablationcharacteristics to occur (24) simultaneously with the best qualityobserver (camera) image (22).

The “mirror-with-hole” invention laser ablation viewing system dependson a second aspect of the generalized laser ablation invention which isthe use of substantially longer-than-normal focal lengths (greater thanF=40 mm, and preferably greater than F=100 mm) in the invention laserobjective focusing lens (8) and substantially longer-than-normalinvention laser object distances (4→8). The substantiallylonger-than-normal invention focal lengths “F”, and substantiallylonger-than-normal invention laser object distances “0” of the inventiongive rise to a substantially longer-than-normal invention laser spotimage distance “I” (8→24) according to the laser objective lens formula:1/F=1/O+1/Iwhere F is the focal length, O is the object distance (4→8) and I is theimage distance (8→24), and this increased invention laser spot imagedistance (I) allows enough room between the invention objective lens (8)and the solid sample surface (11, 24) and sample cell (9, 23) to fit(in) the invention “mirror-with-hole” (25, 26), which could otherwisenot be fitted in (not enough room) below the FIG. 1 conventional priorart analytical laser ablation objective lens (8) which is limited by theprior art short focal length geo-bias.

A third preferred invention aspect, namely that of large focused spotanalysis in SRM analytical laser ablation is also facilitated by thelarger-than-normal focal length invention laser objective lens (8) whichallows a reduction in demagnification ratio, yielding larger inventionSMR homogeneous laser spot diameters—e.g. >0.2 mm and up to 1.5 mm ormore, and this, in turn, permits the use of much larger invention pulsedUV excimer or SRM (frequency multiplied) Nd-YAG lasers (1) greater than12 mJ (@ 266 nm) and, in a non-limiting example, up to 300 mJ at 266 nmwithout exceeding the IIR of solid samples. Up to 25× higher inventionablation rates are thereby enabled, compared to conventional(geo-biased) prior art UV analytical laser ablation, without exceedingthe sample HR. Substantially enhanced invention analytical laserablation sensitivity in the ppb range may thereby be achieved, comparedto reduced (ppm range) sensitivity of prior art analytical laserablation. Embodiments with excimer and 213 nm or 193 nm (frequencymultiplied) Nd-YAG lasers may also be envisioned and these are withinthe scope of the invention, as well as diode pumped lasers, longerwavelength lasers and femto-second lasers. Additional inventionembodiments using a long focal length mirror objective focusing elementmay also be envisioned and are within the scope of this invention.

The longer focal length invention laser ablation objective lens (8)exhibits advantage in providing an opportunity for greater inventionlaser ablation bulk analysis sensitivity of a solid surface, since aninvention longer focal length lens (e.g. F>40 mm, and especially F=150to 400 mm in a nonlimiting example) is naturally accompanied by lessdemagnification in the final SMR focused laser spot size (24), yieldingsignificantly larger invention SMR laser spot diameters (>0.2 mm, andpreferably 0.4-1.6 mm in a nonlimiting example), and therefore allowinguse of more powerful invention excimer and SMR lasers (1) withoutexceeding the IIR and overpowering (e.g. shattering, cracking, largeparticle expulsion, etc.) the sample. Essentially, the same energydensity (joules/cm²) within the sample IIR may be used from a largerinvention laser, but also focused into a larger invention spot diameter(24) to ablate more material, under (desirable) IIR conditions. Theresult is a larger solid sample area is ablated at the same (optimized)energy density by the invention.

The final result is that substantially more sample vapor, smoke, and/orparticulate aerosol is produced within the sample IIR during ablationwith the preferred embodiment large homogeneous spot, high powered,excimer or SMR based Nd-YAG laser ablation invention. Invention ablationrates are therefore desirably stable and consistently higher for thesame sample material and energy density. This gives rise to higherinvention signals in the external analytical instruments (15) to whichthe vapor, smoke, and/or particulate aerosol from the invention aredirected for analysis. The higher signal produced from large area IIRinvention ablation gives rise to enhanced invention analysissensitivity, and the combined invention and external analyticalinstrument (15) are capable of 10-25 fold more analytical sensitivity,depending on how much bigger the invention laser (1) and correspondinglyselected invention laser spot diameter (facilitating ablation within thesample IIR) are chosen to be.

Essentially, the invention employs substantially longer focal lengthlaser objective lenses (8) yielding an option for larger spot diametersin invention excimer and SMR analytical UV laser ablation. Instead oflimiting to 0.2 mm maximum spot diameter, which is the largest focusedspot normally available in conventional prior art excimer and SMRanalytical laser ablation, the invention will allow spot diameters of upto 1.5 mm or more in a nonlimiting example, if a sufficiently largeexcimer or SMR invention laser (1) is substituted to “make up” theformer prior art energy density in the invention larger spot diameters.This will easily allow invention laser ablation analysis in the ppb(parts per billion) or sub-ppb range instead of conventional (part permillion) sensitivity limits of prior art analytical laser ablation.

Referring to FIG. 2A, a preferred embodiment invention excimer or SMRNd-YAG laser (1) is substantially more powerful than correspondinglasers used in prior art analytical laser ablation. This aspect of theFIG. 2A invention analytical laser ablation invention is enabled by theunusually long focal length of invention laser objective lens (8) whichhas focal length greater than F=40 mm (and preferably F=150 mm or morein a nonlimiting example) and is about 4× longer focal length (in anonlimiting example) than prior art excimer or SMR Nd-YAG analyticallaser ablation, and which enables nominally 4× less demagnification andnominally 4× larger focused invention laser spot diameter (24) accordingto the parametric equations (using earlier defined terms):1/F=1/O+1/I and m ⁻¹ =O/I

With nominally 4× larger (nonlimiting example) invention excimer or SMRNd-YAG focused spot diameter (24), the FIG. 2A preferred inventionembodiment can employ a 16× larger SMR invention laser (1) withoutexceeding the ideal irradiance range (IIR in J/cm²/ns) of solid samples.The prior art laser ablation system of FIG. 1A cannot do this, owing toa 4× (or more) shorter focal length prior art objective lens (8) whichdoes not facilitate focused laser spot diameters above 0.2 mm in priorart analytical laser ablation systems using excimer or SMR Nd-YAGlasers.

Further manipulation of invention object and image distances accordingto the above listed parametric equations would actually allow up to a1.5 mm invention spot diameter and a 30× larger invention laser withoutexceeding the IIR of solid samples. The combination of an invention4-30× larger laser with oversized invention spot diameters in the rangeof 0.4-1.5 mm will yield substantially higher ablation rates at typicalsample IIR's and more bulk analysis sensitivity (e.g. 4-30× more) thanprior art excimer or SMR Nd-YAG analytical laser ablation systems.Ultra-trace bulk solids analysis in the parts-per-billion (ppb) rangemay thereby be achieved by a preferred invention embodiment.

By wide range operational repositioning of at least two laser “steering”mirrors (e.g. mirrors 30, 31 in FIG. 2A being moved to alternatepositions in FIGS. 3A-B, 4A, and 5) in a preferred invention foldeddetour laser path coupled with wide range operational repositioning of alaser objective lens (8), a preferred embodiment of the inventionfurther provides for wide range, operationally variable demagnificationratio in the focused invention laser spot size (24). The at least twopreferred invention laser steering mirrors (30, 31) may be manuallyrelocated to alter the length of a preferred invention folded detourlaser object path (FIGS. 2A, 4A, and 5 or FIGS. 3A-3B), or they may bemounted on a preferred embodiment invention precision motorized track(41) with a lead-screw drive (42) as in FIG. 6 for motorized alterationof the preferred invention embodiment folded laser object path length.The invention laser objective lens (to be repositioned) may be manuallyrepositioned or in a preferred FIG. 2B embodiment, it may be mounted ona gantry (35) coupled to a motorized track (36) with a lead-screw formotorized alteration of the overall invention focused laser spotdemagnification ratio in combination with a FIG. 6 motorizedrepositioning or a FIGS. 3A-C manual repositioning of the inventionmirrors 30, 31.

It has been noted that movement of laser steering mirrors 30, 31 in adirection parallel to the beam path from 29-30 in FIGS. 3A-C and inFIGS. 2A, 4A, and 5. In a second embodiment which may function alone, orin combination with parallel movement of mirrors 30, 31, FIGS. 4B-Jillustrate that at least one folded optical detour path may be createdwhich is perpendicular to the original FIG. 2A laser path segmentbetween 31 and 6. By inserting various mirrors 126-128 with aperpendicular motion of wedge mount 125, various folded optical detourpaths including 128→129→134→135 (FIG. 4C), 127→130→133→136 (FIG. 4D),and 126→131→132→137 (FIG. 4E) are enabled which lengthen the pathsegment between 31 and 6 by varied optical detour amounts, while stilldirecting it coincidentally (coaxially) with path 7 through theobjective focusing optic 8 focusing (with demagnification) to solidtarget sample 11. In a third embodiment, FIGS. 4G-J illustrate that apair of single larger mirrors 138, 139 can replace the illustrated sixindividual mirrors 126-128 and 135-137 on the movable wedge mount 125and accomplish essentially the same set of optical detour pathelongations.

Speaking broadly, relocation of the movable wedge mount 125 orrelocation of the at least two invention laser steering mirrors (30, 31)varies the invention object distance, O. Relocation of either the sample(11) or the invention objective lens (8) varies the invention imagedistance I according to the lens formula given earlier. The resultingchanges in O and I then give rise to an alteration of the inventiondemagnification ratio m⁻¹, such that:m ⁻¹ =O/I

The greatest sensitivity for laser ablation analysis for a givenmaterial and a given laser size will occur with the laser operating at100% output power and the full laser beam focused into a spot diameteryielding the ideal irradiance range (IIR) for that sample material andlaser wavelength. Since sample materials vary widely in values of IIR,it would be desirable to have a wide range of full power irradiancevalues available for a single analytical laser ablation system. This isnot possible with prior art laser ablation systems which have a fixedobject distance (O). The lens formula dictates that for a fixed priorart object distance (O) and a fixed prior art focal length (F), theprior art image distance (I) and therefore the prior art demagnificationratio (m⁻¹=O/I) will also be fixed. With a fixed prior artdemagnification value, the irradiance at 100% laser power output willnot vary, and so variations in IIR for different samples may not bematched at full power with a prior art system having fixed O and fixed F(yielding fixed I and fixed m⁻¹). Some samples may fall into the fixedIIR of a given prior art system at full power, but many others will falloutside of their IIR, thus limiting the sensitivity of prior artanalysis, and the reliability of prior art calibration.

Preferred invention embodiments shown in FIGS. 2A-6 solve this problemby allowing substantial practical variation of object distance (O) by asmuch as a full meter or more of path length. Such a large practicalvariation of invention object distance (O) produces a correspondinglylarge variation in invention image distance (I) and inventiondemagnification ratio (m⁻¹), thus enabling the FIGS. 2A-6 preferredinvention embodiments to serve as the first known wide range, variabledemagnification ratio analytical laser ablation system, capable ofablating any solid material within its IIR, and at 100% laser poweroutput, thus achieving maximum sensitivity and calibration reliabilityfor bulk analysis of all materials which is possible for a given laser.To achieve the required large variation in invention object distance,the dichroic mirror pair (30, 31) may be moved right or left in theFIGS. 2A, 4A, 5 and 6 diagrams, thus shortening or lengthening theobject distance in the invention folded detour path or movable wedgemount 125 may be moved up or down in the FIGS. 4B-J diagrams, or acombination of mirror 30, 31 movement and wedge 125 movement may beemployed. A corresponding vertical relocation of sample 11 or inventionobjective lens (8) (or a combination of the two) is needed to satisfythe lens formula (1/F=1/O+1/I) and keep the laser spot image (24)focused at sample surface (11). Invention mirrors 30, 31 and/or wedge125 and sample 11 and/or objective lens 8 are thus positioned tomaintain a focused laser spot image (of aperture 4) on the samplesurface (11, 24). In one preferred invention embodiment, the mirrors 30,31 and/or wedge 125 and objective lens (8) are moved in such a way thatthe lens formula (1/F=1/O+1/I) is always kept satisfied as the focalplane (24) remains fixed. The demagnification ratio (m′=OA) and theirradiance are however greatly altered with these invention mirror andlens movements, and a wide variety of sample IIR may thereby be ideallymatched by the invention.

If desired, the repositioning of invention mirrors (30, 31) and/or wedge125 and invention objective lens (8) may be done manually through use ofinvention kinematic mounts to allow a variety of pre-set inventiondemagnification ratios. If the invention components are kinematicallymounted (39) then a separate set of pre-aligned kinematically mountedinvention mirrors (e.g. 30, 31, 39 in FIGS. 3-5) may be provided foreach different invention path length configuration of an inventionmulti-position (multi-configuration) folded laser path and may bequickly interchanged to effect rapid and convenient operational changeof the invention demagnification ratio, without a system realignment.

It should be noted that vertical motion of lens 8 on a precision motionstage or gantry (see 35, 36 in FIG. 2B) may or may not require inventionlaser system realignment, however motion of the mirror pair 30, 31 willmost certainly require invention laser system realignment to keep thefocused laser spot exactly centered on sample position 24, taken as areference position.

To achieve operational invention laser system realignment uponsubstantial relocation of mirrors 30, 31 and/or lens 8, mirrors 30, 31may be mounted on a plate (39) and plate (39) may be kinematicallymounted to the invention optical platform (40). Pre-alignment ofinvention mirrors 30, 31 for a given plate (39) position on theinvention optical platform (40) will then assure that overall inventionalignment is maintained whenever plate (39) is in it's pre-alignedoptical platform position. A key feature of this preferred inventionembodiment is that plate (39) is only used in one position, so each timeit is installed in position, its kinematic mount ensures that thepre-aligned mirror (30, 31) condition is maintained. To change platepositions (relocation), a different invention plate with a separateinvention mirror pair must then be substituted, with the new mirror pairbeing pre-aligned for the new plate position (also kinematically mountedto the new position). Essentially, this embodiment of the invention usesa new pre-aligned mirror pair and kinematically mounted plate for eachavailable mirror position. To operationally relocate the mirrors, a newmirror pair (and plate) is selected for each position, and each separatemirror pair is pre-aligned to its own location on the optical platform(40). The required number of mirror pairs must equal the required numberof different mirror positions. Operational relocation is achieved simplyby demounting the previous mirror pair (and plate) from itsquick-release kinematic mount, selecting a new mirror pair (pre-alignedfor the new position), and quickly clamping it into its designated (new)position. The pre-alignment characteristic of the newly selected mirrorpair makes it unnecessary to re-align the system upon installation ofthe new pair.

Alternatively, a preferred motorized FIG. 6 embodiment of inventionmirror and objective lens reconfiguration (repositioning) to effectinvention variable demagnification ratio may be computer controlled ifthe motors are precision digital stepping motors. In this case a singlepair of the at least two invention laser steering mirrors (30, 31) wouldbe moved to effect object distance variation in the invention foldedlaser path.

Path length variation by the folded path detours of FIGS. 4B-J have thespecial benefit of having fixed, pre-aligned mirror settings on aprecision gimbaling mount for each mirror in the series 129-134 whichautomatically maintain alignment of image 24 at a preselected referencelocation on sample 11 as wedge 125 is relocated to its various positionswhich are preferred to be kinematically stabilized at each location.Kinematic stabilization ensures that wedge 125 is consistent in itslocations, such that the pre-aligned, preset mirrors 129-134 alwaysensure that image 24 remains centered at the preselected referencelocation on sample 11 as wedge 125 is relocated.

This level of invention system adjustability allows users of aninvention analytical laser ablation system to operationally adjust themaximum focused laser spot size to keep invention focused laser energyand irradiance within the IIR of each and every sample type without needof realignment, regardless of how narrow an individual IIR range may beand how widely the IIR may vary from one material to the next. Whencoupled with the use of larger invention lasers, this inventionoperationally adjustable maximum spot size feature provides for themaximum possible ablation rate, sensitivity and calibration accuracy(and reliability) possible for each sample type, without shattering thesample or exceeding its IIR, even in the face of different materialswith widely varying IIR. Such a characteristic has never before beenavailable in a prior art laser ablation system, and it allows the fullinvention laser power to be 100% utilized in an optimized way on anoperation basis for each sample analysis, and provides forpart-per-billion (ppb) analysis of solid samples by invention UVanalytical laser ablation, instead of the conventional ppm sensitivitylimits of prior art systems.

A further characteristic of a preferred embodiment invention laserablation system operationally variable demagnification ratio feature isautomatic realignment of the invention laser beam following a change ofinvention demagnification ratio. Normally, if folding mirrors wererepositioned to vary the laser path length, a realignment of the lasermirror system would be required. This would normally have to bepainstakingly performed using precision angular adjustment controls forthe mirrors and also using alignment targets and bore sight tooling.Mirror alignments made in one folded path configuration will not holdwhen the folded path is reconfigured (to change its length) byrelocating one or more mirrors.

For a manually reconfigured (FIGS. 2A, 3A-C, 4A, and 5) embodiment ofthe invention variable demagnification feature, the aforementionedpre-aligned mirrors (30, 31) on kinematic mounts (39, FIGS. 3A-C) willsuffice to maintain invention laser beam alignment following a change ofpath length, if a different (separate) invention pre-aligned mirror pair(30, 31) is devoted to each pre-set location in the invention variablefolded detour path. In one non-limiting example, if there are to be 8different invention preset demagnification ratios involving 8 differentmirror pair locations, then 8 different pre-aligned mirror pairs (30,31) would be needed, one (pre-aligned) pair (30, 31) for eachdemagnification ratio to be operationally selected. This requires extramirrors—more than a prior art fixed demagnification system, but theinvention mirror pairs are each pre-aligned, operationally demountable,and kinematically stabilized for precise, quick interchange, so theoperational change of invention demagnification ratio is relatively easyto perform, and requires no system realignment after changing thedemagnification.

An even more convenient (more highly preferred yet) embodiment of theinvention may be envisioned without extra mirrors, if a furthermodification to the preferred FIG. 6 motorized invention embodiment isconsidered. In the preferred FIG. 6 motorized embodiment, the inventionlaser ablation system rapidly achieves automatic laser beam realignmentthrough the folded detour path mirror system, when the at least twoinvention mirrors (30, 31) are relocated to alter the invention foldedpath length, by virtue of invention small precision digitally controlledstepping motors (43) mounted on the precision gimbaling mirror mountscontrolling the invention mirror angles. Preset stepper motor addressesmay be pre-determined (through a pre-alignment exercise) for eachdifferent invention folded detour path length for the laser. Each timethe invention folded path length and demagnification ratio are changedby repositioning the at least two invention mirrors (30, 31), storedvalues of (pre-aligned) invention stepper motor (43) address may beretrieved by the invention computer that correspond exactly to the newalignment angles of the at least two invention mirrors (30, 31) for thenew position, and the invention mirror angles may thus be quickly resetto their new pre-determined alignment for each invention demagnificationratio.

A preferred embodiment of the invention involves actual relocation ofthe same mirror pair 30, 31 to one or more preset locations along aprecision linear track. Precision micrometer settings on the gimbalingmirror angle adjustments of one or both of the two mirrors may bepre-determined to maintain overall system alignment for each presetlocation on the linear track. Pre-determination of mirror gimbalmicrometer settings would be done in a preliminary setup alignmentexercise performed for each preset location on the track. Once a fullset of micrometer settings has been determined (separate settings foreach preset track location), then those micrometer settings simply haveto be replicated (for that track position) each time the mirror pair ismoved to a new location. This may be done manually with precisionmicrometer settings, or digital stepping motors may be attached to thegimbaling adjustments and then the pre-determined stepper motoraddresses set for the gimbaling adjustments on the mirrors correspondingto a given track location selected. Separate stepper motor addresses(mirror gimbaling adjustments) would be predetermined for each presettrack location. A computer may store these stepper motor addresses andthen recall them (and reload them to the stepper motors) each time themirror pair is moved between preset locations.

Invention mirror pair motion to any location between two presetlocations on the linear track may be dealt with by computerinterpolation between the gimbaling stepper motor addresses for thebracketing preset locations. In this way a full range of continuouslyvariable demagnification ratios may be operationally obtained withautomatic system realignment. An invention operator need only enter thedesired magnification ratio into the system computer and a digitalstepping motor will automatically relocate the mirror pair along thelinear track and additional stepping motors will automatically realignthe mirrors to a preset or interpolated alignment corresponding to theselected track position.

In addition, the invention laser objective lens may be positioned on afocus track and controlled by the computer to keep the lens formula(1/F=1/O+1/I) satisfied (image focused) for a fixed sample position, asthe mirrors move. Essentially, when a new demagnification ratio isspecified by the invention user, the computer will solve the parametricequations (1/F=1/O+1/I and m⁻¹=O/I) for a fixed value of F and thespecified m⁻¹ to yield corresponding values of O and I which determinethe mirror and lens placements for that m⁻¹. Then the computer will lookup (or interpolate) new pre-determined pre-alignment values of mirrorgimbaling (angle) adjustments to restore system alignment. Thisinvention feature is completely new to analytical laser ablation and itwill facilitate operational selection of a wide variety ofdemagnification ratios to meet the application-specific IIR requirementsof virtually any solid sample, while allowing the full available laserpower to be used for each analysis. This will maximize inventionsensitivity and also maximize overall analytical instrument calibrationprecision, accuracy, consistency, and reliability.

A further preferred embodiment to extend the range of usable spotdiameters and demagnification ratios would include variable focal lengthin the invention objective lens. To facilitate this, interchangeableinvention objective lenses of varying focal length may be employed,including (in one preferred embodiment) a rotary turret containing atleast two invention objective lenses of different focal length.Invention zoom laser objective lenses and variable focus laser objectivelenses may also be envisioned in other embodiments, either alone, or incombination with other lenses (individually interchangeable or on aturret) so long as they have the requisite UV transmission properties.

In one preferred embodiment, the invention objective lens (or turret)may be mounted on a precision motion stage for repositioning (asinvention laser mirrors are relocated). In another FIG. 6 embodiment(see especially FIG. 2B, items 8, 25, 26, 21, 27, 22), the inventionobjective lens, mirror-with-hole, and visible “white light” achromaticlens and camera may all be mounted on a gantry (35, 66), such that theentire gantry moves to reposition these optics, as invention lasermirrors are relocated.

In one preferred embodiment, the invention gantry may also be preciselymoved (up and down) to focus the laser spot image and camera objectplanes (if coincident) onto the solid sample surface. In anotherembodiment the invention camera (22) may be relocated to shift the whitelight object plane to keep coincident with the laser spot image planewhich may move upon invention laser mirror and laser objective lensrepositioning to achieve varied invention demagnification ratios.

In another preferred embodiment, the solid sample (and/or sample cell)may be moved on a precision vertical motion stage to achieve focus ofthe laser spot image and camera object planes to the sample surface.

Invention modularity may accommodate lasers of widely differing size andpower on a single “flex” platform, without repositioning orreconfiguring the remaining optics.

A final advantage of the “mirror-with-hole” invention laser ablationviewing system is that a prior art thin-film coated dichroic mirror ((6)in FIG. 1) is replaced by the invention mirror-with-hole (25, 26 inFIGS. 2A-B, 9A-B) at an invention optical convergence point of the two(laser and camera) paths, and eliminating the (radiation damage prone)thin-film coating of a prior art dichroic mirror, allows a preferredembodiment invention UV laser ablation mirror-with-hole to functionundamaged for at least 100,000 rads total accumulated radiation exposure(in a nonlimiting example, and e.g. for 100 million rads in a preferredembodiment) in a radiation “hot cell” for analysis of high activitynuclear waste, if the invention laser beam (5, 7) originates outside ofthe “hot cell”. (See FIGS. 7A-B, in which the entire FIGS. 7A-Binvention upper module apparatus is located on top of the hot cell andthe emergent FIGS. 7A-B laser beam (7) proceeds downward into the hotcell through a small opening in the hot cell ceiling), and the inventionfinal line-of-sight mirror (6)—line of sight to a radioactive solidsample in ablation cell (23) of the invention lower module (See FIG. 8A)and also the invention camera (22, See FIGS. 8A-B) are rad-hardenedand/or shielded, respectively. To rad-harden invention line-of-sightmirror 6 (FIGS. 7A-B), it cannot be a prior art thin film dichroic lasermirror (subject to rapid radiation damage), and a fully aluminizedinvention line-of-sight mirror would have to be substituted instead.(Conventional prior art thin film dichroic mirror coatings are rapidlydestroyed by radiation damage at 1,000 rads/hour exposure in anactivated radiation “hot cell”.) The invention aluminized finalline-of-sight laser steering mirror has a reduced reflectance of about96% R when new, compared with a new (non-irradiated) prior art dichroicmirror (99.7% R), but after a short time (e.g. within a few minutes orhours) of exposure to high activity nuclear waste (e.g. 1000 rads/hr),the prior art dichroic mirror will be destroyed and the inventionaluminized final line-of-sight steering mirror will still be 96% R. Asmall percentage reduction (e.g. 3-4%) of initial reflectance in theinvention line-of-sight steering mirror thus extends the inventionuseful lifetime to about 6-12 years, rather than 6-12 minutes (or hours)lifetime for a prior art system. A preferred FIG. 7A-B, 8A-B, 9A-Drad-hardened embodiment of the invention analytical UV laser ablationsystem is thereby enabled for the analysis of solid nuclear waste, or awitness coupon of the nuclear waste which has been vitrified intoradioactive glass, with a small witness coupon to the vitrificationprocess being presented for analysis in ablation cell 23 (11, 24).

One preferred invention embodiment therefore employs a splitarchitecture invention laser ablation system for a radiation hot cell asin FIGS. 7A-B, 8A-B, in which an invention laser (1, FIGS. 7A-B) andinvention laser steering mirrors (29, 30, 31, 6) are located outside ofthe hot cell with a beam (7) from the invention laser entering the hotcell through a window in the hot cell, and in which the invention FIGS.8A-B and 9A-D “lower module” comprising an invention long focal length(uncoated) laser objective lens (8), mirror-with-hole (25, 26),invention uncoated view camera lens (module 66 in FIG. 8A, similar tomodule 66 of FIG. 2B except achromatic lenses (21) are uncoated),invention shielded view camera (67, 22), invention ablation cell (23),invention automated sample changer (44-46, 81-84), invention ablationcell translational motion stages ((44-46, 89, 47-49, 50-52) facilitatingsample focus, line scan ablation, and raster pattern ablation), andinvention energy meter (90) is located inside the hot cell,

In a preferred FIGS. 8A-B embodiment, all hot cell components (keycomponents and subsystems) are modularized for quick dismount andreplacement by a hot cell manipulator arm and gripper claw. Kinematicmounting of invention hot cell laser ablation components and subsystemsis an invention feature which facilitates replacement by manipulatorwith optically pre-aligned replacement components and subsystems,thereby eliminating the need for “manned entry” (along with eliminatingthe need for difficult and expensive hot cell decontamination associatedwith “manned entry”) for the service replacement exercise.

In one preferred embodiment, said invention FIGS. 8A-B “lower module”components in the hot cell are rad-hardened and/or radiation shieldedand/or exhibit placement “at distance” from radioactive samples, topermit each said invention lower module component and the overallinvention lower module to withstand at least 100,000 rads and preferablyup to 100 million rads total lifetime radiation exposure prior to aradiation damage failure point,

and in which additional invention laser ablation components receiving“line of sight” radiation outside the hot cell, such as a finalinvention laser beam steering mirror (6) directing the externalinvention laser beam (7) into the hot cell is rad-hardened to withstandradiation exposure,and in which an invention valve module, directing the flow of carriergas and/or purge gas to the invention ablation cell, is a rad-hardenedvalve module capable of withstanding radiation exposure.

In the various radiation hot cell embodiments so far listed, radhardening is accomplished by the components being manufactured materialsselected from a list prepared and published by nuclear testing groupssuch as CERN, said list being comprised of materials tested and foundnot to deteriorate under accumulated radiation exposure to at least100,000 rads and preferably up to 100 million rads in CERN agencyreports and testing programs.

This includes construction materials, wiring insulation, connectors,cables, motors, lubricants, seals, and optics. The invention uses CERNapproved rad-hardened materials throughout all of its componentsinstalled in the hot cell installation. No prior art laser ablationsystem has done this. Cements and glues are not tolerated. Certainpolymers (e.g. Teflon) are not recommended. Electronics (especiallyintegrated circuit chips) must be outside the hot cell (or heavilyshielded), with only control voltage and current lines entering. Laserablation video cameras and energy meters must be rad-hardened and/orheavily shielded from line-of-sight radiation in the hot cell. All ofthese properties are claimed for the invention laser ablation system, asno prior art laser ablation system employs them, and the invention laserablation system does employ them and an invention prototype has beenbuilt and successfully installed in a radiation hot cell at DOE Hanfordsite, with radiation damage immunity designed to withstand 100 millionrads total lifetime accumulated exposure. At 1000-2000 rads/h, andnormal work shifts, the invention prototype is expected to last 6-12years before failure due to radiation damage. (Prior art laser ablationsystems would last maybe 6-12 minutes, or maybe an hour at most in thisenvironment). Higher or lower levels of rad-hardening may beincorporated and still be included in the scope of this invention.

A final advantage of the “mirror-with-hole” invention laser ablationviewing system is that a conventional prior art thin-film coateddichroic mirror ((6) in FIG. 1) is replaced by the inventionmirror-with-hole (25, 26 in FIG. 9A-B) at an invention opticalconvergence point of the two paths, and eliminating the thin-filmcoating of a prior art dichroic mirror allows a preferred embodimentinvention UV laser ablation to function in a radiation “hot cell” foranalysis of high activity nuclear waste, if the invention laser beam (7)originates outside of the “hot cell” (see FIGS. 7A-B), and the inventionfinal line-of-sight mirror (6)—line of sight to a radioactive solidsample (11, 24) and also the invention camera (22) are rad-hardenedand/or shielded (67), respectively. To rad-harden inventionline-of-sight mirror (6), it cannot be a conventional prior art dichroiclaser mirror (subject to rapid radiation damage), and a fully aluminizedinvention line-of-sight mirror is substituted by the invention instead.(Conventional prior art thin film dichroic mirror coatings are rapidlydestroyed by radiation damage at 1,000 rads/hour exposure in anactivated radiation “hot cell”.) The invention aluminized finalline-of-sight laser steering mirror has a reduced reflectance of about96% R when new, compared with a new (non-irradiated) prior art dichroicmirror (99.7% R), but after a short time (e.g. within a few minutes orhours) of exposure to high activity nuclear waste (e.g. 1000 rads/hr),the prior art dichroic mirror will be destroyed and the inventionaluminized final line-of-sight steering mirror will still be 96% R. Asmall percentage reduction of initial reflectance in the inventionline-of-sight steering mirror thus extends the invention useful lifetimeto about 6-12 years, rather than 6-12 minutes (or hours) lifetime for aprior art system. A preferred FIG. 7A-B, 8A-B rad-hardened embodiment ofthe invention analytical UV laser ablation system is thereby enabled forthe analysis of solid nuclear waste (11, 24).

One preferred invention embodiment employs a split architectureinvention laser ablation system for a radiation hot cell as in FIGS.7A-B, 8A-B in which an invention laser and invention laser steeringmirrors are located outside of the hot cell with a beam from theinvention laser entering the hot cell through a window in the hot cell,and in which the invention “lower module” comprising an invention longfocal length (uncoated) laser objective lens, mirror-with-hole,invention uncoated view camera lens, invention shielded view camera,invention ablation cell, invention automated sample changer, inventionablation cell translational motion stages (facilitating sample focus,line scan ablation, and raster pattern ablation), and invention energymeter are located inside the hot cell,

and in which said invention “lower module” components in the hot cellare rad-hardened and/or radiation shielded and/or exhibit placement “atgreater than normal distance” from radioactive samples, to permit eachsaid invention lower module component and the overall invention lowermodule to withstand at least 100,000 rads and preferably up to 100million rads total lifetime radiation exposure prior to a radiationdamage failure point,and in which additional invention laser ablation components receiving“line of sight” radiation outside the hot cell, such as a finalinvention laser beam steering mirror directing the external inventionlaser beam into the hot cell is rad-hardened to withstand radiationexposure,and in which an invention valve module, directing the flow of carriergas and/or purge gas to and from the invention ablation cell, is arad-hardened valve module capable of withstanding radiation exposure.The invention prototype laser ablation system installed in the hot cellat DOE Hanford site exhibits all of the above embodiment characteristicsand is fully functional.

In preferred FIGS. 9B, 8A-B embodiments of the invention (either “cold”or rad-hardened) laser ablation system, a demountable sample ablationcell for laser ablation analysis is employed in which the ablation cellcomponents assemble and seal by vertically stacking (mating) components,without using fasteners, tie downs, latches, clamps, snaps, bolts or anyother fastener or clamping means. Assembly and low pressure sealing issimply by stacking the mated components vertically, and demounting issimply by unstacking the components (with simple “lift off” means),without need to remove or release any fastener, latch, or clamp. In apreferred embodiment invention demountable sample ablation cell, gasseals are achieved by a weight compression factor, with upper cellcomponents having sufficient weight to deliver a gas sealing force tomating lower cell components. The seals or a combination of seals areselected from among a group comprising tapered seals, gaskets, ando-rings and in which the selected seals are compressed to their sealingpoints solely by the weight of stacked overhead cell components.

If the weight of stacked overhead cell components becomes excessive, apreferred FIG. 8A-B embodiment of the invention employs a demountablesample ablation cell (23, 76, 80) in which a counterbalancing force(95-102) is applied in compound linkages and levers to offset thecombined weight of stacked ablation cell components (23) and FIG. 9B(all) without diminishing sealing forces below their gas sealing points,in order to allow “light duty” X, Y, Z translational stages to controlthe combined stacked cell positioning. The counterbalancing force mayinvolve a spring loaded plate or platform, or it may involve at leastone counterbalancing weight.

In a preferred invention embodiment, an invention sample changer forlaser ablation analysis may cause samples or sample holders (containingsamples) to be sequentially placed in proximity to an ablation cell toeffect sequential laser ablation events and sample analysis by anexternal ICP, ICP-MS, or FAG-MS instrument, in which a sample changingmeans places at least a first sample in proximity to an ablation cell,and in which said sample changing means removes said first sample afterlaser ablation analysis, and in which said sample changing means thenplaces at least a second sample in proximity to said ablation cell.

In a preferred embodiment sample changer, samples may be sequentiallylifted out of a counter bore in a movable platform (83, see FIG. 9A)selected from a movable platform group comprising a rotary carousel, anR-Theta rotating/sliding tray, an X, Y sliding tray, or a linearfeed-through tray or conveyor, said samples or sample holders(containing samples) being lifted out of said movable platform by amechanized push rod (81, 106) which pushes upward through a through-hole((124)—FIG. 9C, (24)—FIG. 8A) contained within the counterbore, andlifts the samples (11) or sample holders (82, containing samples 11) upand out of the movable platform 83, and in which the lifting actionfurther places the samples or sample holders in proximity to a laserablation sample cell as in FIG. 9C.

In a preferred embodiment a segment (81) of the push rod o.d. diameteris less than the i.d. of the through hole (124) in the movable platform,to an extent which allows horizontal motion of the push rod to effect aline scan, or x, y raster scan, or R-Theta raster scan of the samplehorizontally in the laser beam. The invention sample changer's movableplatform sequentially presents the samples or sample holders (containingsamples) of a group “one at a time” for the push rod to sequentiallylift into proximity to the laser ablation sample cell, so that eachsample may be analyzed sequentially (in turn) by laser ablationanalysis. Referring to FIGS. 9A-C, the sample changer lifting actionseals the sample or sample holder (82) (containing a sample (11))against or into a sample ablation cell (23) via weight-stacked matchingtapers (an o.d. taper (82) on the sample holder mating to an identicali.d. taper (108) in the base of the sample cell).

The sample changer may continue push rod (81) lifting action aftersealing to further lift the sample cell and sample or sample holder(containing a sample) as a stack, said lift proceeding upward to liftthe stack out of a stationary sample ablation cell holding platform (75)and further continues the lift until the upper surface of the samplereaches a laser ablation focal plane (24) or a specified defocused laserablation plane. The mechanized push rod and lift stage is furthermounted atop an X, Y (47-49, 50-52) or R-Theta translational stagecapable of offsetting the push rod with stacked sample holder, sample,and sample ablation cell in a linear horizontal motion (see FIG. 9Doffset (24, 11, 75-76, 47-48) or an X, Y horizontal raster pattern, oran arc motion or an R-Theta raster pattern for laser ablation or toselected stationary horizontal offset positions for laser ablation afterlifting and focusing.

In another FIG. 10 preferred embodiment, the invention sample changermay keep the support rod (81) vertically stationary and employ themovable platform (83) to position a sample over the support rod and thenlower the sample or sample holder (containing sample) onto the supportrod and the platform continues to lower after the sample holder engagesthe top of the support rod, such that the platform lowers itself toclear the bottom edge of the sample or sample holder. In this embodimentit is preferred that invention laser focusing is be performed byvertical rise or fall of an invention overhead gantry (66) containing atleast the laser objective lens (8). In a preferred embodiment, theinvention gantry would also support the invention visible white lightviewing system and mirror-with-hole (25, 26—see FIG. 2B). In a preferredFIG. 10 embodiment, the gantry also functions to raise or lower thesample ablation cell enclosure (23) over the stationary sample or sampleholder (containing sample).

In another embodiment, an invention sample cell for laser ablation hasthe sample cell closed on the top and open on the bottom, and in whichthe open bottom is positioned in proximity to a sample surface, and inwhich carrier gas enters the cell via the annular space between thebottom of the sample cell and the top of the sample surface, and inwhich an outer concentric “skirt” affixed to the sample cell o.d.provides a compliant seal to the sample, and in which carrier gas isentered into the annular space from the skirt. In this embodiment, thesample cell is horizontally stationary, but the compliant seal is asliding seal which allows the sample to move horizontally withoutbreaking the seal. In one embodiment, the i.d. of the bottom of theinvention sample cell and skirt are both smaller than the perimeter ofthe sample, such that the compliant seal is formed to the samplesurface. In another embodiment, the i.d. of at least the skirt is largerthan the perimeter of the sample, such that the compliant seal is formedto the sample holder.

In an alternate embodiment, the compliant seal is an inflatable anddeflatable bladder which may be deflated for change of sample andinflated to re-establish perimeter seal around the sample. In thisembodiment, the samples are presented sequentially in an x, y slidingtray or rotary platter, or R-theta platter during inflate/deflate cyclesto effect an inexpensive automatic sample changer.

BRIEF DESCRIPTION OF THE DRAWINGS

The Foregoing and other aspects, benefits and advantages of theinvention will be better understood from the following detaileddescription of preferred embodiments of the invention with reference tothe drawings, in which:

FIG. 1A is an unscaled 2-dimensional block diagram of a prior artanalytical laser ablation system.

FIG. 1B. is a truncated block diagram section of FIG. 1A, enlarged toshow greater detail and a clearer view of prior art items 8-13, 16-19,23 and 24 which appear in both figures, with addition of a set of X, Y,Z motorized translational stages (44-53).

FIG. 2A is an unscaled 2-dimensional diagram of an embodiment of theinvention laser ablation large laser (1), mirror-with-hole (25, 26), andlong objective focal length system (8).

FIG. 2B is a 3-D front view (at slightly elevated vertical perspective)preferred invention embodiment drawing showing a mirror-with-hole (25,26) and an optical gantry (35, 36, 66) containing objective lens (8),mirror-with-hole (25, 26), sample cell lift hooks (121), and achromaticwhite light path (33, 21, 27, 20) with camera (22). Gantry (35, 36, 66)raises and lowers to stack or unstack the stackable sample ablation cell(23) onto post ((38)—same as (81) in FIG. 10) by means of tabs (54) andlift hooks (121).

FIGS. 3A-C are rear view block diagrams of FIG. 2B front view preferredinvention embodiment drawing showing invention variable laser pathlength (5).

FIG. 4A is an unscaled block diagram similar to FIG. 2A, with laser path(4-7) variation by moving 2 mirrors (30-31).

FIG. 4B is the same as FIG. 2A with addition of movable wedge mount(125) on which are is mounted six beam steering laser mirrors (126-128and 135-137) which do not intercept the laser beam segment between 31and 6 in this figure. Six more laser beam steering mirrors (129-134) areillustrated in fixed locations along paths at right angles to the laserbeam segment between 31 and 6 in this figure. None of the twelve addedlaser beam steering mirrors are active in this figure and the laser beamsegment length from 31 to 6 is unaffected.

FIG. 4C is the same as FIG. 4B except for the wedge mount (125) havingmoved upward in the figure, inserting mirror 128 into the laser beam,which reflects the beam 90 degrees upward to mirror 129 which receivesthe beam and reflects it over to mirror 134 which reflects it down againto mirror 135 which restores the beam to a path coincident with itsoriginal path leading into mirror 6. The path length from 31 to 6 hasbeen increased by an optical detour path from 128→129→34→135 with theincreased path increment essentially equal to twice the offset of mirror129 from 128, but the final laser beam segment emerging from the opticaldetour continues on the normal trajectory into mirror 6.

FIG. 4D is the same as FIG. 4C except for the wedge mount 125 havingmoved further upward in the figure, removing mirrors 128, 129, 134, and135 from the laser beam path and replacing them in the laser beam pathwith mirrors 127, 130, 133, and 136 enabling a longer optical detourpath from 127→130→133→136 with the optical detour path incrementessentially equal to twice the offset of mirror 130 from 127, but thefinal laser beam segment emerging from the optical detour continues onthe normal trajectory into mirror 6.

FIG. 4E is the same as FIG. 4D except for the wedge mount 125 havingmoved further yet upward in the figure, removing mirrors 127, 130, 133,and 136 from the laser beam path and replacing them in the laser beampath with mirrors 126, 131, 132, and 137 enabling a longer opticaldetour path yet from 126→131→132→137 with the optical detour pathincrement essentially equal to twice the offset of mirror 131 from 126,but the final laser beam segment emerging from the optical detourcontinues on the normal trajectory into mirror 6.

FIG. 4F is the same as FIG. 4E except that mirrors 30 and 31 have movedsubstantially to the right in the figure, shortening the basic opticalpath segment from 31 to 6 and making another four optical path lengthsfrom 31 to 6 possible with four-position relocation of wedge mount 125as earlier illustrated in FIGS. 4B-E but using the new positions ofmirrors 31 and 32. FIGS. 4B-F have thus illustrated eight differentoptical path lengths obtainable within a single laser ablation system,simply by moving either wedge mount 125 (FIGS. 4B-E) or mirrors 31, 30.

FIG. 4G is the same as FIG. 4E except that individual mirrors 126-128have been replaced with a single larger mirror 138, and individualmirrors 135-137 have been replaced with a single larger mirror 139.

FIG. 4H is the same as FIG. 4G except that movable wedge mount has beenrelocated somewhat lower in the figure so that the incident laser beam(from 31) intercepts mirror 138 at a different location on the mirror,causing it to reflect to mirrors 130, 133, and 139 instead of mirrors131, 132, and 139. The FIG. 4H optical detour path has been decreasedaccordingly from what it was in FIG. 4G.

FIG. 4I is the same as FIG. 4H except that movable wedge mount has beenrelocated somewhat lower yet in the figure so that the incident laserbeam (from 31) intercepts mirror 138 at a different location yet on themirror, causing it to reflect to mirrors 129, 134, and 139 instead ofmirrors 130, 133, and 139. The FIG. 4I optical detour path has beendecreased accordingly from what it was in FIG. 4H.

FIG. 4J is the same as FIG. 4I except that movable wedge mount has beenrelocated even lower yet in the figure so that the incident laser beam(from 31) no longer encounters an optical detour path and it proceedswithout detour to mirror 6.

FIG. 5 is the same as FIG. 4A, but with greater movement of the mirrorpair 30, 31 and shorter overall optical path length.

FIG. 6 is a motorized version of FIGS. 2A, 4A, and 5.

FIG. 7A is a 3-D angled perspective view of an upper module (outside ofradiation hot cell) of a rad-hardened laser ablation invention.

FIG. 7B is a shorter path version of FIG. 7A with fewer mirrors and 2energy meters (58).

FIG. 8A is a lower module (inside radiation hot cell) of rad-hardenedlaser ablation invention.

FIG. 8B is the same as 8A but with shield (67) removed, and withaddition of a counterbalancing force (95) to offset weight of theablation cell (23).

FIG. 9A. is another invention version of the FIG. 8A preferredembodiment.

FIG. 9B. is an exploded view of a stacking ablation cell (23) from FIG.9A.

FIG. 9C shows the sample holder lifted out of its platform (83) engagingthe ablation cell and lifting it out of its platform (75), focusingsample 11 at objective lens focal plane (24).

FIG. 9D is the same as 9C except platform 47 has motor scanned to theright, effecting a line scan of the laser beam to the left on sample 11,24.

FIG. 10 illustrates a laser ablation autosampler added to a FIG. 2Blaser ablation system with platter 83 lowering over stationary supportrod 81 and gantry 66 having already lowered stacking ablation cell (23)onto stationary support rod (81) and tapered plug (106) (see FIG. 9A.).The FIG. 10 autosampler is an R-Theta type, in which rotary carousel 83also translates linearly on track 123 to select among the threeconcentric rings containing sample holders 82.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 2A-B, a preferred embodiment of the inventioninvolves a mirror-with-hole (25, 26) positioned below long focal lengthinvention laser objective lens (8). The invention mirror-with-hole (25,26) allows a focused invention UV laser beam (7, 10) to pass (unaltered)through the hole (26) to the solid material surface (11) while theinvention observer (22) visible “white light view” (28) of said solidmaterial surface is obtained off axis with the invention mirrorperimeter (33) essentially concentrically surrounding the hole (26) andsaid UV laser beam (7, 10) passing through said hole (26). The advantageof this invention is that a final delivery segment (to the solid samplesurface (11)) of the invention UV laser beam (10) is coaxiallysuperimposed with an initial segment (18) of the invention visible“white light” observer view (22) with both invention paths sharing asingle coincident focal plane (24), which is the “image” plane of theinvention laser objective lens (8) and is also the “object” plane of theinvention achromatic white light camera lens doublets (21), but withoutthe two invention paths sharing any common steering or focusing optic,thus effecting optical “decoupling” of the invention UV laser beam (10)from the invention visible “white light” observer view (18, 20, 22),even though the two invention light paths coaxially share a superimposedpath segment (10, 18). The focal length of invention UV laser ablationobjective lens (8) is longer than conventional prior art analyticallaser ablation objective lens focal lengths and the invention longerobjective lens focal length creates “working room” under said inventionlaser objective lens (8) which allows room for the invention “mirrorwith hole” (25, 26) to fit in under said invention objective lens,without interfering with the invention sample ablation cell (23) or itswindow (9).

Invention optical decoupling of the two paths is desirable for UV laserablation because a UV laser focusing lens ((8) if refractive, andregardless of quality) is not an ideal, aberration-free viewing opticfor visible “white light” observer or camera viewing (22). Conversely,an achromatic lens (21) designed for high quality “white light” viewingby an observer (or camera (22)) is not suited to UV laser focusing (ahigh quality visible white light achromatic lens being typically made ofglass (or plastic) and therefore opaque to UV laser light). Themirror-with-hole (25, 26) invention optically decouples the laser path(10) from the observer (or camera) path ((18, 20, 22) no shared opticalsteering or focusing elements), and allows completely separate(individually optimized) focusing optics (8 versus 21) to be used foreach invention path, though an invention path segment (10, 18) istraversed by both invention beams, and it specifically provides a higherquality achromatic “white light” view (22) of the solid material surfacebefore, during, and after an invention UV laser ablation event. Sharperinvention white light images of the sample surface (11, 24) aretherefore seen by the observer or camera (22), while a high qualityinvention UV laser objective lens (8) produces a high quality laser spoton the sample (11), to effect the best ablation characteristics with theinvention. The best UV ablation is thus obtained by the invention, whilesimultaneously yielding the best quality white light view of the event.

Referring to FIG. 2A, a preferred embodiment invention excimer or SMRNd-YAG laser (1) is substantially more powerful than correspondinglasers used in prior art analytical laser ablation. This aspect of theFIG. 2A invention analytical laser ablation invention is enabled by theunusually long focal length of invention laser objective lens (8) whichhas focal length greater than F=40 mm (and preferably F=150 mm or morein a nonlimiting example) and is about 4× longer focal length (in anonlimiting example) than prior art excimer or SMR Nd-YAG analyticallaser ablation, and which enables nominally 4× less demagnification andnominally 4× larger focused invention laser spot diameter (24) accordingto the parametric equations (using earlier defined terms):1/F=1/O+1/I and m ⁻¹ =O/I

With nominally 4× larger (nonlimiting example) invention excimer or SMRNd-YAG focused spot diameter (24), the FIG. 2A preferred inventionembodiment can employ a 16× larger SMR invention laser (1) withoutexceeding the ideal irradiance range (IIR in J/cm²/ns) of solid samples.The prior art laser ablation system of FIG. 1A cannot do this, owing toa 4× (or more) shorter focal length prior art objective lens (8) whichdoes not facilitate focused laser spot diameters above 0.2 mm in priorart commercially available analytical laser ablation systems usingexcimer or SMR Nd-YAG lasers.

Further manipulation of invention object and image distances accordingto the above listed parametric equations would actually allow up to a1.5 mm invention spot diameter and a 30× larger invention laser withoutexceeding the IIR of solid samples in a nonlimiting example. The(nonlimiting) combination of an invention 4-30× larger laser withoversized invention spot diameters in the (nonlimiting) range of 0.4-1.5mm will yield substantially higher ablation rates at typical sampleIIR's and more bulk analysis sensitivity (e.g. 4-30× more) than priorart excimer or SMR Nd-YAG analytical laser ablation system. Ultra-tracebulk solids analysis in the parts-per-billion (ppb) range may thereby beachieved by a preferred invention embodiment.

The greatest sensitivity for laser ablation analysis for a givenmaterial and a given laser size will occur with the laser operating at100% output power and the full laser beam focused into a spot diameteryielding the ideal irradiance range (IIR) for that sample material andlaser wavelength. Since sample materials vary widely in values of IIR,it would be desirable to have a wide range of full power irradiancevalues available for a single analytical laser ablation system. This isnot possible with prior art laser ablation systems which have a fixedobject distance (O). The lens formula dictates that for a fixed priorart object distance (O) and a fixed prior art focal length (F), theprior art image distance (I) and therefore the prior art demagnificationratio (m⁻¹=O/I) will also be fixed. With a fixed prior artdemagnification value, the irradiance at 100% laser power output willnot vary, and so variations in IIR for different samples may not bematched at full power with a prior art system having fixed O and fixed F(yielding fixed I and fixed m⁻¹). Some samples may fall into the fixedIIR of a given prior art system at full power, but many others will falloutside of their IIR, thus limiting the scope and sensitivity of priorart analysis, and the reliability of prior art calibration.

A first preferred invention embodiment shown in FIGS. 2A, 3A-C, 4A, 5and 6, a second preferred embodiment shown in FIGS. 4B-J, and/or a thirdpreferred embodiment combining the first and second preferredembodiments solves this problem by allowing substantial practicalvariation of object distance (O) by as much as a full meter or more ofpath length. Such a large practical variation of invention objectdistance (O) produces a correspondingly large variation in inventionimage distance (I) and invention demagnification ratio (m⁻¹), thusenabling the FIGS. 2A, 3A-C, 4A, 5, and 6 first preferred inventionembodiment and the FIGS. 4B-J second preferred embodiment and/or a thirdpreferred embodiment combining the first and second embodiments to serveas the first known wide range, variable demagnification ratio analyticallaser ablation system, capable of ablating any solid material within itsIIR, and at 100% laser power output, thus simultaneously achievingmaximum sensitivity and calibration reliability for bulk analysis allmaterials which is possible for a given laser. To achieve the requiredlarge variation in invention object distance, the dichroic mirror pair(30, 31) may be moved right or left in the FIGS. 2A, 4A, 5 and 6diagrams, thus shortening or lengthening the object distance in theillustrated invention folded detour path.

It has been noted that movement of laser steering mirrors 30, 31 in adirection parallel to the beam path from 29-30 in FIGS. 3A-C and inFIGS. 2A, 4A, and 5. In a second embodiment which may function alone, orin combination with parallel movement of mirrors 30, 31, FIGS. 4B-Jillustrate that at least one folded optical detour path may be createdwhich is perpendicular to the original FIG. 2A laser path segmentbetween 31 and 6. By inserting various mirrors 126-128 with aperpendicular motion of wedge mount 125, various folded optical detourpaths including 128→129→134→135 (FIG. 4C), 127→130→133→136 (FIG. 4D),and 126→131→132→137 (FIG. 4E) are enabled which lengthen the pathsegment between 31 and 6 by varied optical detour amounts, while stilldirecting it coincidentally (coaxially) with path 7 through theobjective focusing optic 8 focusing (with demagnification) to solidtarget sample 11. In a third embodiment, FIGS. 4G-J illustrate that apair of single larger mirrors 138, 139 can replace the illustrated sixindividual mirrors 126-128 and 135-137 on the movable wedge mount 125and accomplish essentially the same set of optical detour pathelongations.

In summary, to achieve the required large variation in invention objectdistance, the dichroic mirror pair (30, 31) may be moved right or leftin the FIGS. 2A, 4A, 5 and 6 diagrams, thus shortening or lengtheningthe object distance in the illustrated invention folded detour path.Alternatively the movable wedge mount (125) may be moved up or down inthe FIGS. 4B-J diagrams, or a combination of right/left movement of thedichroic mirror pair (30-31) and the up/down movement of the movablewedge (125) may be employed.

A corresponding vertical relocation of invention objective lens (8)and/or the solid target sample 11 is needed to satisfy the lens formula(1/F=1/O+1/I) and keep the laser spot image (24) focused at samplesurface (11). Invention mirrors (30, 31) and objective lens (8) are thuspositioned to maintain a focused laser spot image (24) (of aperture 4)on the sample surface (11). In a preferred invention embodiment, themirrors (30, 31) and objective lens (8) are moved in such a way that thelens formula (1/F=1/O+1/I) is always kept satisfied as the focal plane(24) remains fixed. The demagnification ratio (m⁻¹=O/I) and theirradiance are however greatly altered with these invention mirror andlens movements, and a wide variety of sample IIR may thereby be ideallymatched by the invention.

It should be noted that vertical motion of lens 8 on a precision motionstage may or may not require invention laser system realignment, howevermotion of the mirror pair 30, 31 will most certainly require inventionlaser system realignment to keep the focused laser spot (24) exactlycentered on sample position (11), taken as a reference position.

To achieve operational invention laser system realignment uponsubstantial relocation of mirrors (30, 31), they may be mounted on aplate (39) and plate (39) may be kinematically mounted to the inventionoptical platform (40). Pre-alignment of invention mirrors (30, 31) for agiven plate (39) position on the invention optical platform (40) willthen assure that overall invention alignment is maintained wheneverplate (39) is in the given pre-aligned optical platform (40) position. Akey feature of this preferred invention embodiment is that a given plate(39) is only used in one position, so each time it is installed in theone position, its kinematic mount ensures that the pre-aligned mirror(30, 31) condition is maintained. To change plate positions(relocation), a different invention plate with a separate inventionmirror pair must then be substituted, with the new mirror pair beingpre-aligned for the new plate position (also kinematically mounted tothe new position). Essentially, this embodiment of the invention uses anew pre-aligned mirror pair and kinematically mounted plate for eachavailable mirror position. To operationally relocate the mirrors, a newmirror pair (and plate) is selected for each position, with eachseparate mirror pair having been pre-aligned to its own location on theoptical platform (40). The required number of mirror pairs must equalthe required number of different mirror positions. Operationalrelocation is achieved simply by demounting the previous mirror pair(and plate) from its quick-release kinematic mount, selecting a newmirror pair (pre-aligned for the new position), and quickly clamping itinto its designated (new) position. The pre-alignment characteristic ofthe newly selected mirror pair makes it unnecessary to re-align thesystem upon installation of the new pair. An alternate embodiment wouldhave each prealigned mirror of the selected pair on separate kinematicmounts instead of on plate (39).

A preferred embodiment of the invention involves actual relocation ofthe same mirror pair 30, 31 to one or more preset locations along aprecision linear track (FIG. 6). Precision micrometer settings on thegimbaling mirror angle adjustments (43) of one or both of the twomirrors may be pre-determined to maintain overall system alignment foreach preset location on the linear track. Pre-determination of mirrorgimbal micrometer settings would be done in a preliminary setupalignment exercise performed for each preset location on the track. Oncea full set of micrometer settings has been determined (separate settingsfor each preset track location), then those micrometer settings simplyhave to be replicated (for that track position) each time the mirrorpair is moved to a new location. This may be done manually withprecision micrometer settings, or digital stepping motors may beattached to the gimbaling adjustments and then the pre-determinedstepper motor addresses set for the gimbaling adjustments on the mirrorscorresponding to a given track location selected. Separate stepper motoraddresses (mirror gimbaling adjustments) would be predetermined for eachpreset track location.

In a preferred embodiment, a computer may store these stepper motoraddresses and then recall them (and reload them to the stepper motors)each time the mirror pair is moved between preset locations.

Invention mirror pair motion to any location between two presetlocations on the linear track may be dealt with by computerinterpolation between the gimbaling stepper motor addresses for thebracketing preset locations. In this way a full range of continuouslyvariable demagnification ratios may be operationally obtained withautomatic system realignment. An invention operator need only enter thedesired magnification ratio into the system computer and a digitalstepping motor will automatically relocate the mirror pair along thelinear track and additional stepping motors will automatically realignthe mirrors to a preset or interpolated alignment corresponding to theselected track position.

In addition, the invention laser objective lens (8) may be positioned ona focus track and controlled by the computer to keep the lens formula(1/F=1/O+1/I) satisfied (image focused) for a fixed sample position, asthe mirrors move. Essentially, when a new demagnification ratio isspecified by the invention user, the computer will solve the parametricequations (1/F=1/O+1/I and m⁻¹=O/I) for a fixed value of F and thespecified m⁻¹ to yield corresponding values of O and I which determinethe mirror (30, 31) and lens (8) placements for that m⁻¹. Then thecomputer will look up (or interpolate) new pre-determined pre-alignmentvalues of mirror gimbaling (angle) adjustments (43) to restore systemalignment. This invention feature is completely new to analytical laserablation and it will facilitate operational selection of a wide varietyof demagnification ratios to meet the application-specific IIRrequirements of virtually any solid sample, while allowing the fullavailable laser power to be used for each analysis. This will maximizeinvention sensitivity and also maximize overall analytical instrumentcalibration precision, accuracy, consistency, and reliability.

Path length variation by the folded path detours of FIGS. 4B-J have thespecial benefit of having fixed, pre-aligned mirror settings on aprecision gimbaling mount for each mirror in the series 129-134 whichautomatically maintain alignment of image 24 at a preselected referencelocation on sample 11 as wedge 125 is relocated to its various positionswhich are preferred to be kinematically stabilized at each location.Kinematic stabilization ensures that wedge 125 is consistent in itslocations, such that the fixed, pre-aligned, preset mirrors 129-134always ensure that image 24 remains centered at the preselectedreference location on sample 11 as wedge 125 is relocated.

A further preferred embodiment to extend the range of usable spotdiameters and demagnification ratios would include variable focal lengthin the invention objective lens. To facilitate this, interchangeableinvention objective lenses of varying focal length may be employed,including (in one preferred embodiment) a rotary turret containing atleast two invention objective lenses of different focal length.Invention zoom laser objective lenses and variable focus laser objectivelenses may also be envisioned in other embodiments, either alone, or incombination with other lenses (individually interchangeable or on aturret) so long as they have the requisite UV transmission properties.

In one preferred embodiment, the invention objective lens (or turret)may be mounted on a precision motion stage for repositioning (asinvention mirrors are relocated). In another embodiment, the inventionobjective lens (8), mirror-with-hole (25, 26), and visible “white light”achromatic lens (21) and camera (22) may all be mounted on a FIG. 2Bgantry, such that the entire gantry (66, 35, 36) moves to repositionthese optics, as invention laser mirrors (30, 31) are relocated as inFIGS. 2A, 3A-C, and FIGS. 4A, 5 and 6 and/or as the movable wedge mount(125) is relocated as in FIGS. 4B-J.

In one preferred embodiment, the invention gantry may also be preciselymoved (up and down) to focus the laser spot image and camera objectplanes (if coincident) onto the solid sample surface. In anotherembodiment the invention camera may be relocated to shift the whitelight object plane to keep coincident with the laser spot image planewhich may move upon invention laser mirror and laser objective lensrepositioning to achieve varied invention demagnification ratios.

In another preferred embodiment, the solid sample (11) and/or sampleablation cell (23) may be moved on a precision vertical motion stage toachieve focus of the laser spot image (24) and camera object planes (28)to the sample surface (11).

Invention modularity may accommodate lasers of widely differing size andpower on a single “flex” platform, without repositioning orreconfiguring the remaining optics.

A final advantage of the “mirror-with-hole” invention laser ablationviewing system is that a conventional prior art thin-film coateddichroic mirror ((6) in FIG. 1A-B) is replaced by the inventionmirror-with-hole (25, 26) at an invention optical convergence point ofthe two paths (see FIGS. 2A-B, FIGS. 9A-B), and eliminating thethin-film coating of a prior art dichroic mirror allows a preferredembodiment invention UV laser ablation (FIGS. 7A-B, 8A-B) to function ina radiation “hot cell” for analysis of high activity nuclear waste, ifthe invention laser beam (7) originates outside of the “hot cell” (seeFIGS. 7A-B), and the FIG. 7A-B invention final line-of-sight mirror(6)—line of sight to a FIG. 9A (also FIG. 8A-B) radioactive solid sample(11, 24) and also the invention camera (22) are rad-hardened and/orshielded, respectively. To rad-harden invention line-of-sight mirror(6), it cannot be a conventional prior art dichroic laser mirror(subject to rapid radiation damage), and a fully aluminized inventionline-of-sight mirror would have to be substituted instead. (Conventionalprior art thin film dichroic mirror coatings are rapidly destroyed byradiation damage at 1,000 rads/hour exposure in an activated radiation“hot cell”.) The FIG. 7A-B invention aluminized final line-of-sightlaser steering mirror (6) has a reduced reflectance of about 96% R whennew, compared with a new (non-irradiated) prior art dichroic mirror(99.7% R), but after a short time (e.g. within a few minutes or hours)of exposure to high activity nuclear waste (e.g. 1000 rads/hr), theprior art dichroic mirror will be destroyed, but the inventionaluminized final line-of-sight steering mirror will still be 96% R. Asmall percentage reduction of initial reflectance in the inventionline-of-sight steering mirror (6) thus extends the invention usefullifetime to about 6-12 years, rather than 6-12 minutes (or hours)lifetime for a prior art system. A preferred FIGS. 7A-B, 8A-B, FIGS.9A-D rad-hardened embodiment of the invention analytical UV laserablation system is thereby enabled for the analysis of solid nuclearwaste (11, 24).

One preferred invention embodiment employs a split architectureinvention laser ablation system for a radiation hot cell as in FIGS.7A-B, 8A-B, in which FIGS. 7A-B invention laser, invention upper energymeters (58) and invention laser steering mirrors are located outside ofthe hot cell with a beam from the invention laser entering the hot cellthrough a window in the hot cell, and in which the FIGS. 8A-B invention“lower module” comprising an invention long focal length (uncoated)laser objective lens, mirror-with-hole, invention uncoated view cameralens, invention shielded view camera, invention ablation cell, inventionautomated sample changer, invention ablation cell translational motionstages (facilitating sample focus, line scan ablation, and rasterpattern ablation), and invention lower energy meter (90) is locatedinside the hot cell,

and in which said FIGS. 8A-B invention “lower module” components in thehot cell are rad-hardened and/or radiation shielded and/or exhibitplacement “at greater than normal distance” from radioactive samples, topermit each said invention lower module component and the overallinvention lower module to withstand at least 100,000 rads and preferablyup to 100 million rads total lifetime radiation exposure prior to aradiation damage failure point,and in which additional invention laser ablation components receiving“line of sight” radiation outside the hot cell, such as a finalinvention laser beam steering mirror directing the external inventionlaser beam into the hot cell is rad-hardened to withstand radiationexposure,and in which an invention valve module, directing the flow of carriergas and/or purge gas to and from the invention ablation cell, is arad-hardened valve module capable of withstanding radiation exposure.

In preferred embodiments of the invention (either “cold” orrad-hardened) laser ablation system, a demountable sample ablation cellfor laser ablation analysis is employed in which the ablation cellcomponents assemble and seal by vertically stacking (mating) components,without using fasteners, tie downs, latches, clamps, snaps, bolts or anyother fastener or clamping means. Assembly and sealing is simply bystacking the mated components vertically, and demounting is simply byunstacking the components (with simple “lift off” means), without needto remove or release any fastener, latch, or clamp. In a preferredembodiment invention demountable sample cell, gas seals are achieved bya weight compression factor, with upper cell components havingsufficient weight to deliver a sealing force to mating lower cellcomponents. The seals or a combination of seals are selected from amonga group comprising tapered seals, gaskets, and o-rings and in which theselected seals are compressed to their gas sealing points solely by theweight of stacked overhead cell components.

If the weight of stacked overhead cell components becomes excessive, aFIG. 8B preferred embodiment of the invention employs a demountablesample cell in which a counterbalancing force (95) is applied to offsetthe combined weight of stacked cell (23) components without diminishingsealing forces below their gas sealing points, in order to allow “lightduty” X, Y, Z translational stages to control the combined stacked cellpositioning. The counterbalancing force may involve a spring loadedplate or platform, or it may involve at least one counterbalancingweight (95).

In a preferred invention embodiment, an invention sample changer forlaser ablation analysis may cause samples or sample holders (containingsamples) to be lifted out of a counter bore in a movable platformselected from a movable platform group comprising a rotary carousel, anR-Theta rotating/sliding tray, an X, Y sliding tray, or a linearfeed-through tray or conveyor, said samples or sample holders(containing samples) being lifted out of said movable platform by amechanized push rod which pushes upward through a through-hole (or otheropening) contained within the counterbore, and lifts the samples orsample holders (containing samples) up and out of the movable platform,and in which the lifting action further places the samples or sampleholders in proximity to a laser ablation sample cell.

In a preferred embodiment a segment of the push rod o.d. diameter isless than the i.d. of the through-hole (or opening) in the movableplatform, to an extent which allows horizontal motion of the push rod toeffect a line scan, or X, Y raster scan, or R-Theta raster scan of thesample horizontally in the laser beam. The invention sample changer'smovable platform sequentially presents the samples or sample holders(containing samples) of a group “one at a time” for the push rod tosequentially lift into proximity to the laser ablation sample cell, sothat each sample may be analyzed sequentially (in turn) by laserablation analysis. In one embodiment, the sample changer lifting actionseals the sample or sample holder (containing a sample) against or intoa sample cell via weight stacked matching tapers (an o.d. insertingtaper on the sample holder mating to an identical i.d. receiving taperin the base of the sample cell).

The sample changer may continue push rod lifting action after sealing tofurther lift the sample ablation cell and sample or sample holder(containing a sample) as a stack, said lift proceeding upward to liftthe stack out of a stationary sample cell holding platform and furthercontinues the lift until the upper surface of the sample reaches a laserablation focal plane or a specified defocused laser ablation plane. Themechanized push rod and lift stage is further mounted atop an X, Y orR-Theta translational stage capable of offsetting the push rod withstacked sample holder, sample, and sample ablation cell in a limitedlinear horizontal motion or a limited X, Y horizontal raster pattern, ora limited arc motion or a limited R-Theta raster pattern duringrepetitively firing laser ablation events or to selected stationaryhorizontal offset positions for laser ablation after lifting andfocusing.

In another FIG. 10 preferred embodiment, the invention sample changermay keep the push rod (81) vertically stationary and employ the movableplatform (83) to position a sample over the push rod and then lower thesample or sample holder (82) (containing sample) onto the push rod andthe platform continues to lower after the sample engages the top of thepush rod, such that the platform lowers itself to clear the bottom edgeof the sample or sample holder. In this embodiment it is preferred thatinvention laser focusing is be performed by vertical rise or fall of aninvention overhead gantry (66) containing at least the laser objectivelens. In a preferred embodiment, the invention gantry would also supportthe invention visible white light viewing system and mirror-with-hole.In a preferred embodiment, the gantry also functions to raise or lowerthe sample ablation cell enclosure (23) over the stationary sample bymeans of lifting/lowering hooks (121) engaging/disengaging lift tabs(54) on the ablation cell (23).

In another embodiment, an invention sample ablation cell for laserablation has the sample ablation cell closed on the top and open on thebottom, and in which the open bottom is positioned in proximity to asample surface, and in which carrier gas enters the ablation cell viathe annular space between the bottom of the sample ablation cell and thetop of the sample surface, and in which an outer concentric “skirt”affixed to the sample ablation cell o.d. provides a compliant seal tothe sample, and in which carrier gas is entered into the annular spacefrom the skirt. In this embodiment, the sample ablation cell ishorizontally stationary, but the compliant seal is a sliding seal whichallows the sample to move horizontally without breaking the seal. In oneembodiment, the i.d. of the bottom of the invention sample ablation celland skirt are both smaller than the perimeter of the sample, such thatthe compliant seal is formed to the sample surface. In anotherembodiment, the i.d. of at least the skirt is larger than the perimeterof the sample, such that the compliant seal is formed to the sampleholder.

In yet another alternate embodiment, the compliant seal is an inflatableand deflatable bladder which may be deflated for change of sample andinflated to re-establish perimeter seal around the sample. In thisembodiment, the samples are presented sequentially in an x, y slidingtray or rotary platter, or R-theta platter during inflate/deflate cyclesto effect an inexpensive automatic sample changer.

The figures and description are of nonlimiting examples, and the laserablation invention may be envisioned beyond the scope of specificembodiments described herein, and the scope of the invention musttherefore be considered to be limited only by the claims. While theinvention has been described in terms of its preferred embodiments,those skilled in the art will recognize that the invention can bepracticed with modification within the spirit and scope of the appendedclaims.

What is claimed is:
 1. A target viewing system for laser ablation inwhich applications of the target viewing system are selected from thegroup consisting of laser ablation analysis of solid target samples,rad-hardened laser ablation analysis of high-activity radioactive solidtarget samples in a radiation hot-cell, rad-hardened laser ablationanalysis of low-activity radioactive solid target samples in a radiationhot-cell, and laser micro-machining of solid target materials, and inwhich a target illuminator illuminates an area of interest on the targetindependently of the laser beam which independently ablates the targetwithin its illuminated area of interest, and in which illumination lightrays reflected from the illuminated target and viewed by the targetviewing system are essentially coaxial with a final segment of the laserablation beam immediately preceding the target, but in which the targetview is not taken through any laser focusing optic, and in which thelaser focus is thereby optically decoupled from the target viewingsystem focus, and in which the target is selected from the groupconsisting of a solid target sample to be analyzed by laser ablation, ahigh-activity radioactive solid target sample to be analyzed byrad-hardened laser ablation in a radiation hot-cell, a low-activityradioactive solid target sample to be analyzed by rad-hardened laserablation in a radiation hot cell, and a solid target material to bemicro-machined, and in which the laser ablation beam passes through anopening in an angled target viewing mirror in which the opening isselected from the group consisting of a round hole, an elliptical hole,and a slot, and in which the illumination light rays initially reflectedfrom the illuminated target comprise the primary target view rays, andin which the target view rays are secondarily reflected toward a viewingsystem selected from among the group consisting of a viewing camera, aviewing projector, and a viewing eyepiece, and in which the secondaryreflection of the target view rays occurs from an area of the angledtarget viewing mirror selected from the group of areas consisting of anarea which is essentially concentric to the perimeter of the opening, anarea which essentially surrounds the perimeter of the opening.
 2. Thetarget viewing system of claim 1, in which the angled target viewingmirror is located at a placement selected from the group of placementsconsisting of placement between a final laser focusing optic and thetarget, placement in which the final laser focusing optic is located atleast partially within the opening in the target viewing mirror, andplacement in which the final laser focusing optic protrudes at leastpartially through the opening in the target viewing mirror.
 3. Thetarget viewing system of claim 1 in which passing the laser beam throughthe opening in the angled target viewing mirror enables the angledtarget viewing mirror (with the opening) to replace a conventional(prior-art) dichroic laser mirror which is otherwise normally used inprior-art laser ablation to simultaneously reflect a prior-art laserablation beam to a solid target material for purposes of ablating thesolid target material while simultaneously transmitting prior-artwhite-light primary target-view rays from the solid target material to aprior-art view camera, and in which invention replacement of theprior-art dichroic laser mirror eliminates radiation-damage-proneprior-art dichroic laser mirrors from the invention target viewingsystem, and in which elimination of radiation-damage-prone prior-artdichroic laser mirrors from the invention target viewing systemessentially imparts a degree of rad-hardening to the invention targetviewing system, and in which the degree of rad-hardening imparted to theinvention target viewing system enables it to serve as a robust targetviewing system for laser ablation analysis of radioactive solid targetsamples in a radiation hot-cell.
 4. The target viewing system of claim 3in which an added reflector intercepts the secondarily reflectedtarget-view rays before they reach the viewing camera, and in which theadded reflector reflects the secondarily reflected target-view raysbehind a radiation shield to the viewing camera which is also locatedbehind the radiation shield, and in which at least the target viewingsystem, the radioactive solid target sample, the final laser focusingoptic, the added reflector, the radiation shield, and the viewing cameraare all located within a radiation hot-cell, and in which the targetviewing system operates robustly in the radioactive environment.
 5. Thetarget viewing system of claim 4 in which at least one achromaticoptical condensing element focuses the secondarily reflected target-viewrays to form an image of the solid target sample onto the viewingcamera.
 6. The target viewing system of claim 1 in which a surfacecontour of the angled target viewing mirror (with opening) is selectedfrom the group consisting of a flat, a toroid, and a curved mirror. 7.Variable demagnification laser ablation system in which the laserablation system applications are selected from the group consisting oflaser ablation analysis of solid samples and laser micro-machining ofsolid materials, and in which the variable demagnification laserablation system comprises a laser, at least two beam-steering lasermirrors, and a laser-focusing objective optic, and in which a focusedbeam of the laser produced by the laser-focusing objective optic removes(by ablation at focused laser beam irradiance exceeding a surface damagethreshold of a solid target in which the solid target is selected fromthe group consisting of a solid sample to be analyzed by laser ablationand a solid material to undergo laser micro-machining, and in whichlaser ablation occurs within a laser spot of the focused laser beamimpinging on the solid target placed in proximity to an image plane ofthe laser spot produced by the laser-focusing objective optic, and inwhich micro-machining occurs within a laser spot of the focused laserbeam impinging on the solid target placed in proximity to an image planeof the laser spot produced by the laser-focusing objective optic) aportion from the surface of the solid target, for purposes selected fromthe group consisting of altering the shape of the solid target, alteringthe topography of the solid target, and obtaining a plume in which theplume is selected from at least one among the group consisting of aplume of vapors, a plume of smoke, and a plume of particulate aerosolfrom a laser ablation event occurring on the surface of the solidtarget, and in which the plume may injected into a flowing carrier gasstream leading from the solid target contained within an ablation cellto an external analytical instrument selected from among the group ofexternal analytical instruments consisting of an inductively coupledplasma (ICP) emission spectrometer, an inductively coupled plasma massspectrometer (ICP-MS), and a flowing after-glow (FAG) mass spectrometer(FAG-MS), and in which the external analytical instrument is capable ofproviding analysis of the plume, in which the analysis is selected froma group consisting of chemical analysis and elemental analysis, and inwhich the analysis of the plume is representative of the composition ofthe original solid target, and in which changes in the extent of opticaldemagnification of the laser spot used to ablate the target may be madeon an operational basis for purposes of altering the focused laser beamirradiance of the laser spot impinging on the solid target to optimizeparameters selected from the group consisting of target ablation rate,aerosol quality, aerosol particle size, target ablation crater size,target ablation crater quality, target ablation trench size, and targetablation trench quality within the ideal irradiance range (IIR) of eachsolid target, for at least two different solid targets exhibiting atleast two different ideal irradiance ranges, and in which the extent ofoptical demagnification of the laser spot is nominally determined as ademagnification ratio L₁/L₂ of a first laser optical path length (L₁) toa second laser optical path length (L₂), in which the first laseroptical path length (L₁) is the length of a first segment of the laserbeam occurring between a region proximal to the laser head and a regionproximal to the laser-focusing objective optic, and in which the secondlaser optical path length (L₂) is the length of a second segment of thelaser beam occurring between the laser-focusing objective optic and thesolid target placed in proximity to an image plane (IP) of thelaser-focusing objective optic, and in which a lens formula isessentially obeyed (1/IP+1/L₁=1/F), where F is the effective focallength of the laser-focusing objective optic), and in which variation inthe extent of optical demagnification of the laser spot is achieved byaltering the first laser optical path length (L₁), and in which alteringthe first laser optical path length (L₁) produces a shift in thelocation of the image plane (IP) of the laser spot plus a correspondingalteration of the length of the second laser optical path (L₂) accordingto the lens formula, and in which the solid target is relocated proximalto the shifted image plane IP of the laser spot, and in which thealtered extent of optical demagnification of the laser spot is nominallydetermined as the new ratio of the altered first laser optical pathlength (L₁) to the altered second laser optical path length (L₂), and inwhich at least two different first laser optical path lengths (L₁) areselectable in the same laser ablation system thereby creating a choiceof at least two different demagnification ratios (L₁/L₂), and in whichselection between the at least two different first laser optical pathlengths is made by a unidirectional equidistant translational offset ofthe positions of the at least two beam-steering laser mirrors, and inwhich the unidirectional equidistant translational offset is selectedfrom the group of offsets consisting of a longitudinal offset parallelto the incident laser beam and a lateral offset perpendicular to theincident laser beam, and in which the at least two beam-steering lasermirrors are placed in a segment of the laser beam corresponding to thefirst laser optical path, and in which the unidirectional equidistanttranslational offset of the positions of the at least two beam-steeringlaser mirrors occurs within the segment of the laser beam correspondingto the first laser optical path essentially altering the first laseroptical path length L₁.
 8. The variable demagnification laser ablationsystem of claim 7 in which, for the case of the longitudinal offset(parallel to the incident laser beam), a first beam-steering lasermirror of the at least two beam-steering laser mirrors is angled atessentially 45 degrees (e.g., 45 degrees clockwise, in a nonlimitingexample) to a portion of the laser beam incident on the firstbeam-steering laser mirror thereby producing essentially a 90 degreeprimary reflection (e.g., 90 degrees clockwise, in a nonlimitingexample) of the laser beam, and in which a second beam-steering lasermirror of the at least two beam-steering laser mirrors is positioned toreceive the beam reflected from the first beam steering laser mirror,and in which the second beam-steering laser mirror is angled atessentially 45 degrees (e.g., 45 degrees clockwise, in a nonlimitingexample) to the 90 degree primary reflected laser beam thereby producingan additional 90 degree secondary reflection (e.g., an additional 90degrees clockwise, in a nonlimiting example) of the laser beam, and inwhich the secondarily reflected laser beam is traveling essentiallyparallel to the incident beam (but in the opposite direction), and inwhich the optical path traced by the incident portion of the laser beam,the segment of the laser beam immediately following primary 90 degreereflection, and the segment of the laser beam immediately followingsecondary 90 degree reflection essentially traces the shape of arectangular box with only three sides showing, and in which thesimultaneous unidirectional equidistant translational offset of thepositions of the at least two beam-steering laser mirrors used to alterthe first laser optical path length (L₁) occurs essentially parallel tothe incident portion of the laser beam and in a direction selected fromamong a group of directions consisting of directly toward the incidentbeam origin (thereby shortening the first laser optical path length (L₁)by an amount essentially twice the offset) and directly away from theincident beam origin (thereby lengthening the first laser optical pathlength (L₁) by an amount essentially twice the offset, and in which thesegment of the laser beam immediately following secondary 90 degreereflection after altering the first laser optical path length remainscoincident with its original path (before altering the first laseroptical path length (L₁)).
 9. The variable demagnification laserablation system of claim 8, in which the at least two beam-steeringlaser mirrors are mounted on a common base, and in which the common basemay be relocated to produce the simultaneous unidirectional equidistantlongitudinal translational offset of the positions of the at least twobeam-steering laser mirrors within the segment of the laser beamcorresponding to the first laser optical path.
 10. The variabledemagnification laser ablation system of claim 9, in which an opticalmount of at least one of the at least two beam-steering laser mirrors isan adjustable gimbaling mount, and in which the common base relocationto produce the simultaneous unidirectional equidistant longitudinaltranslational offset of the positions of the at least two beam-steeringlaser mirrors is accompanied by at least one readjustment of the atleast one adjustable gimbaling mount to correct for any overall laserbeam misalignment which may have occurred as a result of the common baserelocation and accompanying translational offset, and in which the atleast one readjustment restores laser beam alignment of the first laseroptical path segment after common base relocation to a condition ofcoincidence with overall laser beam alignment before common baserelocation.
 11. The variable demagnification laser ablation system ofclaim 10, in which the at least one readjustment of the at least oneadjustable gimbaling mount is performed using a method selected from thegroup of methods consisting of manual readjustment based on restoringthe focused laser spot position to a predetermined reference targetposition, manual readjustment to a predetermined precision readjustmentsetting which has been predetermined to consistently restore the focusedlaser spot position to a predetermined reference target position, andautomated readjustment by a computer algorithm controlling a precisionstepper motor which performs motorized readjustment to a predeterminedprecision readjustment setting which has been predetermined toconsistently restore the focused laser spot position to a predeterminedreference target position.
 12. The variable demagnification laserablation system of claim 9, in which the common base supporting the atleast two laser mirrors comprises a first base sub-assembly, and inwhich at least a second common base supporting at least two additionallaser mirrors comprises a second base sub-assembly, and in which thefirst base sub-assembly is kinematically mounted to a fixed laserablation platform in a first position, and in which the second basesub-assembly is kinematically mounted to the fixed laser ablationplatform in a second position which is offset from the first position,and in which the first base sub-assembly can be removed from itskinematic mount and replaced to its kinematic mount without need ofrealignment of its laser mirrors such that original laser alignment ismaintained and reproduced upon first sub-assembly replacement to itsfirst kinematic mount receiver, and in which the second basesub-assembly can be removed from its second kinematic mount and replacedto its second kinematic mount without need of realignment of itsadditional laser mirrors such that original laser alignment ismaintained and reproduced upon second sub-assembly replacement to itssecond kinematic mount receiver, in a manner and in which only one ofthe at least two common bases is installed at a time, and in selectionbetween which of the at least two common bases is installed at any giventime will determine which of the corresponding at least two differentlaser spot demagnification ratios will be selected, and in whichselection between the at least two different laser spot demagnificationratios is achieved simply by selecting which of the at least two commonbases to install in their respective locations, and in which installinga new common base does not require mirror realignment, and in whichinstalling a new common base produces the same effect as relocation of asingle common base to produce the simultaneous unidirectionalequidistant translational offset of the positions of the at least twobeam-steering laser mirrors to produce a desired change of laser spotdemagnification.
 13. The variable demagnification laser ablation systemof claim 7 in which, for the case of the lateral offset (perpendicularto the incident laser beam), at least two more beam steering mirrors areadded to create a set of at least four beam steering mirrors which yieldpath-lengthening, four-step optical detour for the incident laser beam,and in which the four-step optical detour initially leads the reflectedlaser beam essentially perpendicularly away from the incident laser beampath in a primary reflection by the first laser beam steering mirror ofthe at least four laser beam steering mirrors, eventually turning thelaser beam essentially parallel again to the incident laser beam path(proceeding in the same direction as the incident laser beam (but withan offset)) in a secondary reflection by the second beam steering lasermirror of the at least four beam steering laser mirrors, and thenturning the laser beam essentially perpendicularly toward the incidentlaser beam path in a tertiary reflection by the third laser beamsteering mirror of the at least four laser beam steering mirrors, andfinally turning the laser beam essentially perpendicularly again in aquaternary reflection by the fourth laser beam steering mirror of the atleast four laser beam steering mirrors, and in which the laser beam(following quaternary reflection) is on a path which is essentiallycoaxial (without offset) with a path which the incident laser beam wouldhave followed if the four step optical detour had not occurred, and inwhich the alteration of path length L₁ introduced by the four-stepoptical detour is essentially twice the distance between the first andsecond laser beam steering mirrors of the at least four laser beamsteering mirrors.
 14. The variable demagnification laser ablation systemof claim 13 in which the second and third laser beam steering mirrorsare maintained in a fixed position while the first and fourth laser beamsteering mirrors of the at least four laser beam steering mirrors may bemechanically positioned on an operational basis to a location selectedfrom the group of locations consisting of in the laser beam or removedfrom the laser beam, and in which the location in the laser beam enablesthe four step path-lengthening optical detour, and in which the locationremoved from the laser beam eliminates the four step path-lengtheningoptical detour such that choice of two different L₁ path lengths isoffered by selecting location of the first and fourth laser beamssteering mirrors of the at least four laser beam steering mirrors to beeither in the laser beam or removed from the laser beam.
 15. Thevariable demagnification laser ablation system of claim 14 in which thefirst and fourth laser beam steering mirrors are mounted on a movableplatform which inserts them into the laser beam enabling the four-stepoptical detour when the platform is moved in one direction perpendicularto the incident laser beam, and in which the first and fourth laser beamsteering mirrors are removed from the laser beam disabling the four-stepoptical detour when the platform is moved in the opposite direction, andin which the movable platform contains at least one additional pair oflaser beam steering mirrors oriented at the same angles as the first andfourth laser beam steering mirrors but with a different spacing betweenthem than is exhibited by the first and fourth laser beam steeringmirrors, and in which the at least one additional pair of laser beamsteering mirrors is offset in the direction of the four stage opticaldetour path, and in which the extent of relocation of the movableplatform determines whether the first and fourth laser beam steeringmirrors are inserted into the laser beam or whether the at least oneadditional pair of laser beam steering mirrors is inserted into thelaser beam, and in which insertion of the first and fourth laser beamsteering mirror pair enables a first optical detour path length based onthe first, second, third, and fourth laser beam steering mirrors, and inwhich insertion of the at least one additional pair of laser beamsteering mirrors enables a second optical detour path length based onthe at least one additional pair of laser beam steering mirrors plus atleast a second additional pair of laser beam steering mirrors which havea different optical detour path offset from the laser beam than thesecond and third laser beam steering mirrors of the at least four laserbeams steering mirrors and in which the at least a second additionalpair of laser beam steering mirrors has a spacing between the two laserbeam steering mirrors of the at least a second pair of additional beamsteering mirrors equivalent to the spacing between the two laser beamsteering mirrors of the at least one additional pair of laser beamsteering mirrors, and in which the extent of relocation of the movableplatform determines the optical path length to be selected from amongthe group of optical path lengths consisting of at least the firstoptical detour path, the second optical detour path, and no opticaldetour.
 16. The variable demagnification laser ablation system of claim15 in which at least the first optical detour path and the secondoptical detour path are nested, one within the other.
 17. The variabledemagnification laser ablation system of claim 15 in which at least thefirst laser beam steering mirror of the at least four laser beamsteering mirrors and the first-encountered laser beam steering mirror ofthe at least one additional pair of laser beam steering mirrors arecombined into a single larger mirror, and in which at least the fourthlaser beam steering mirror of the at least four laser beam steeringmirrors and the second laser beam steering mirror of the at least oneadditional pair of laser beam steering mirrors are combined into asingle larger mirror, and in which relocation of the movable platformwithin the laser beam selects at least two different portions of thecombined first-encountered single larger mirror (combined from the firstlaser beam steering mirror of the at least four laser beam steeringmirrors and the first-encountered laser beam steering mirror of the atleast one additional pair of laser beam steering mirrors) and at leasttwo different portions of the combined second-encountered single largermirror (combined from the fourth laser beam steering mirror of the atleast four laser beam steering mirrors and the second-encountered laserbeam steering mirror of the at least one additional pair of laser beamsteering mirrors).
 18. A long focus laser ablation system in which thelaser ablation system applications are selected from the groupconsisting of laser ablation analysis of solid samples and lasermicro-machining of solid materials, and in which the long focus laserablation system comprises a laser, at least two beam-steering lasermirrors, and a laser-focusing objective optic, and in which a focusedbeam of the laser produced by the laser-focusing objective optic removes(by ablation at focused laser beam irradiance exceeding a surface damagethreshold of a solid target in which the solid target is selected fromthe group consisting of a solid sample to be analyzed by laser ablationand a solid material to undergo laser micro-machining, and in whichlaser ablation occurs within a laser spot of the focused laser beamimpinging on the solid target placed in proximity to an image plane ofthe laser spot produced by the laser-focusing objective optic, and inwhich micro-machining occurs within a laser spot of the focused laserbeam impinging on the solid target placed in proximity to an image planeof the laser spot produced by the laser-focusing objective optic) aportion from the surface of the solid target, for purposes selected fromthe group consisting of altering the shape of the solid target, alteringthe topography of the solid target, and obtaining a plume in which theplume is selected from at least one among the group consisting of aplume of vapors, a plume of smoke, and a plume of particulate aerosolfrom a laser ablation event occurring on the surface of the solidtarget, and in which the plume may injected into a flowing carrier gasstream leading from the solid target contained within an ablation cellto an external analytical instrument selected from among the group ofexternal analytical instruments consisting of an inductively coupledplasma (ICP) emission spectrometer, an inductively coupled plasma massspectrometer (ICP-MS), and a flowing after-glow (FAG) mass spectrometer(FAG-MS), and in which the external analytical instrument is capable ofproviding analysis of the plume, in which the analysis is selected froma group consisting of chemical analysis and elemental analysis, and inwhich the analysis of the plume is representative of the composition ofthe original solid target, and in which the laser is selected from thegroup consisting of a pulsed ultraviolet laser, a stable multimoderesonator (SMR) configured pulsed laser, a picosecond laser, and afemtosecond laser, and in which the laser focusing objective opticexhibits a focal length of at least 40 mm, and in which the focal lengthis sufficient to create an increased working distance between the laserfocusing objective optic and a target focal plane of the laser focusingobjective optic.
 19. The long focus laser ablation system of claim 18 inwhich the working distance is sufficient to allow room for an angledtarget viewing mirror, in which the angled target viewing mirror has anopening through which a focused laser ablation beam from the final laserfocusing optic may pass, and in which the working distance is sufficientto allow additional room for a target sample ablation chamber beneaththe angled target viewing mirror.
 20. The long focus laser ablationsystem of claim 18, in which an optical characteristic of a laser spotproximal to the target focal plane is a depth-of-focus of at least 0.4mm, and in which the at least 0.4 mm depth of focus enables consistentablation rates of a target under conditions of irregular target surfacetopography so long as the degree of irregularity in target surfacetopography doesn't exceed the depth of focus, and in which the degree towhich a mounted target surface is parallel to a translational stagemotion may be relaxed to an extent limited by the depth of focus and theunidirectional length of a trench being ablated into the target byrepetitive, spatially overlapping, pulsed laser ablation event firingsoccurring during simultaneous translational stage motion whichtranslates a sample ablation cell containing the target.